Here’s the answer: “Never.”
Why?
Ask any music teacher what single element sets apart the struggling musician and the accomplished one, and 99% of the time the answer will be RHYTHM.
Music is about flow, about pacing, about sounds created in harmony and rhythm with one another.
If you train the notes into your ear without the intended rhythm, you’ve trained them in with an invented rhythm. If you’ve done it 7 times, your brain now “knows” the piece that way. Do you know how many times you’ll have to do it right to retrain your brain? Thirty five. That’s right, 35!!
Okay, so you’re going to practice smart and get the rhythm BEFORE you learn the notes. Now what?
Feel the Heart Beat
If you are new to music, or if you never “paid attention” to rhythm and know it is a weak spot, you probably have trouble feeling and keeping the pulse. Your metronome is your new best friend. Practice counting and clapping with your metronome until you can walk into another room, out of earshot, still clapping, and then come back and find that you are still in time with the metronome.
Practice keeping the beat along with a recording of your piece, or along with any recorded music, for that matter. Get in the habit of tapping your foot, and you can learn to do that while you're playing (this is especially useful if you want to play with other people).
Make Your Music Count
Learn rhythm inside and out. This isn't as hard as it sounds. Rhythm has an alphabet and it’s not very big. Remember that the subdivisions of each beat have to add up. If a dotted note has “stolen” half of beat two, the eighth that comes next will fall on the “and” of 2.
Practice playing rhythm by clapping and counting all sorts of rhythmic patterns. This will allow you to learn rhythm by feeling it in your body on a deep, automatic level.
Rhythm notation is a fairly simple system, and, like, pitch notation, just takes a little dedicated effort to learn. The payoff? You’ll be playing music that sounds like music.
Connect the Dots
You know how counting works, you’ve written in “1 & 2 & 3 & 4 &” in every measure, and you still can’t count and tap? You’ve probably written your counting in without aligning every number or symbol with the correct note. This little problem can completely derail your efforts to learn rhythm. Write your counting straight down the middle of each measure, with each symbol precisely placed, and you will find counting while tapping or playing so much easier.
Be Silly
Try this: learn rhythm by practicing with words instead of numbers. Suzuki piano teachers use “plum” for a quarter note, “apple” for 2 eighth notes, “watermelon” for 4 sixteenth notes, and “cantaloupe” for the 3 notes of a triplet. Practice counting measures of plums and apples, fitting the apples into the same amount of times as a plum. If you don’t like the fruit idea, use “da” for quarters and “dee” for eighths. Dee dee dah dee dee dah . . .
You may feel silly, but you already know how to say two syllables in the space of one, so you can easily learn how to tap (and play) two notes in the space of one. From there, every other possibility follows. Make it fun, with your own silly words, and learn rhythm at the same time!
Breathe and Glide through the Long Notes
Never, ever shorten a long note or ignore a rest. Listen to singers, and imagine how it would sound if those glorious long notes ended abruptly, replaced by what comes next. What would happen if they never took a breath between phrases? Don’t be in such a hurry to place and play the next measure that you cheat the one that came before--you have more time than you think. Play at a consistent pace and let those spaces breathe for exactly as long as they’re supposed to. The best shortcut? Singing! Even if you "can't carry a tune", singing as you play will teach you how to play more musically as well as help you learn rhythm.
Spot the Troublemaker
If you keep starting at the beginning of your piece, every time, you will keep stumbling at the same spots. This makes it especially difficult to learn rhythm correctly. If you want to make the piece flow, the shortest route is to use practice spot techniques to make the trouble spot better and better--all by itself--so that it improves.
Be Tap Happy
Look at the hardest part of your piece. Set your metronome to a tempo slow enough that you can count and clap that practice spot (and any others). Once your are through learning the rhythm of hard parts, use the same metronome setting and count and tap the soundboard of your harp all the way through, first the right hand's part and then the left. When that is easy, use two hands to tap both the left hand and right hand parts together as you count. You will find that you have to pick an even slower tempo.
Next, try playing one hand while tapping the other. If that's a struggle, often it helps to have the right hand "learn" the left hand's part. Somehow, once the right hand "knows" the pattern, the left hand has an easier time.
When you can do that, try playing the whole piece while counting out loud, still using the metronome.
Theory Pages - Students are to follow the procedures set forth in the examples. First, write the dashes, dots or numbers, then, drop them down to arrows or letters. Also, remember to put parentheses around all rests. It is suggested that the first three exercises be assigned to the students as homework. When graded, if the student receives 100%, he/she passes out of that level. If the student does not, he/she is to take the director's corrections home, and try again doing # 4, 5 & 6. If the student still can not complete the assigned exercises correctly, he/she should receive individual tutoring before trying to complete #7, 8, & 9. Directors also have the "Blank Page" option. This is where they can add their own counting system of choice to the examples on the theory pages as well as the "Say" pages. These pages will then be inserted into their own personal copies of whatever Rhythm Masters books are ordered for your school or school system. 
"Down Clap" and "Down touch Up touch" Drills - This allows the students to actively feel subdivision as it relates to pulse In the initial stages, the act of involving the full body, not just the foot, helps create a dependency on using subdivision in order to count rhythms correctly. As the students become more skilled through practice, the need for full body involvement becomes less essential. Eventually, the foot alone will be able to take on the role of establishing both the pulse and subdivision when counting rhythms. 
Usage B: The students read the exercise in the book. After establishing a steady tempo, the director selects one of the exercises and holds up the appropriate number of fingers. He then calls out "one, two, ready, begin" and the students will begin counting the selected rhythm aloud. After the class is counting the first selected rhythm correctly, the director holds up a different number of fingers to indicate the new rhythm to be counted. He then calls out "one, two, ready, change" and, when the students hear this, they are to look up at the conductor to see to which rhythm to change. This helps develop the skill of looking up at the director and then back at the music. 
Say" and "Play" Drills - After establishing the tempo the director wishes to use, the students will first say the counting out loud reading the left page, and then play the same rhythm on their instrument, reading the right page. Both pages will normally end with a half note, followed by a half rest, allowing the students time to bring their instrument up to playing position, as well as back down to their laps for the "Say" portion of this drill. Once the rhythms in this chapter have been analyzed and rehearsed by saying and playing them, fold the left page back so only the "Play" page is showing. Re-establish a similar tempo used in the first drill, and this time have the students play the rhythms straight down the page reading music only. This drill will improve your students "rhythm vocabulary" in the same way a student first learning to read evolves from a "phonics" to a "whole language" recognition of words in learning to read. For any rhythms causing difficulties, go back to the previous page and review the problem rhythms by saying the counting first. Re-demonstrate the written analysis on the board if needed. 
Power Grids - The "Power Grid" is designed to give your students the opportunity to fully demonstrate their reading skills by playing the new rhythms learned in the present chapter in a totally random order. In this way, the students cannot get "comfortable" with what it sounds like through drill repetition, and are forced to really read the music presented to them. The "Power Grids" also are a great quick evaluation tool to test or quiz your students on their reading ability. The grids should be played in the following order: Day 1, lines #1 - 8; Day 2, columns A - H; Day 3, repeat the process if neccessary, or even use this opportunity to teach retrograde motion . When your students seem to have mastered the rhythms learned in that chapter, move on to sections D or E or the next chapter. If your students are having difficulty with playing the "Power Grid", go back to the "Play Drills" or the "Say and Play Drills" and work your way back up again. 
Song Reinforces - These may be out of range of your first year players due to the additional notes used, but it is great for review the second year. You could offer these exercises for extra credit for your more advanced students to help motivate them to practice the skill of looking up and learning new note fingerings. The first three songs are titled for your students. The last two or three are "Mystery Songs". The idea is for your students to use the rhythms learned earlier in that chapter, and apply them in order to solve the secret identity of each mystery song. The director may even give a "pop quiz", asking the students to write down the names of mystery songs A, B, & C. 
Challenge Drills - Most students today are totally dependent upon the standard beaming of the pulse in order to read rhythms accurately. We still occasionally run into musicthat is not beamed in a standard fashion (for whatever the reason), which will initially baffle most performers, and yes, even some of us band directors. The concept being, if a student can read music that is purposely miss-beamed, then they can read ANYTHING!!!! at that level. Use these drills for second year review, or as extra credit studies with your first year students.