I did practice plans. I didn't care about the number of minutes. I had a form that the kids used to PLAN their practice. How to set goals for the session and week, warm up, focus on a few tasks, spend some time on fun stuff. If their practice session was 5 minutes or 35 minutes I told them time mattered less than approach.

The first one they turned in was an automatic 100 and we did it in class. I helped them write the first goals and day 1. Then they filled out the plan for the rest of the week. I gave them a 6 week planner at a time after those first few. The beginning of each week I would give them 2 or 3 global goals and they came up with a couple of personal goals for the week. Global might be a new scale or a section of the music.
Personal goals might be region or a portion of their solo or ensemble. Keep it attainable and measurable as success leads to success. Most of the kids did it because it was a matter more of filling it out and less about the time and that seemed to help. Plus the good kids really learned how to practice efficiently and isn't that the purpose anyway? The kids that faked it faked it but the kids that did it saw tremendous growth.

How many times do we tell kids to practice but we never TEACH them to practice. And we have a lot of kids that practice but never get better because bad practice might be worse than no practice. At least in class we are controlling the approach and habits....

And for those of you that think sports are our mortal enemy I learned this approach (then modified it for band) not from a band director, (even though my directors were all very wonderful directors and teachers), but from a college golf coach.