I think 4.5 to 6 hours a week in High School is good. That would give you 1.5 to 2 hours 3 times a week. In college you should be doing 12 to 15 hours a week regularly. That's 3 hour pratices 4 or 5 days a week. Exercises? I like the F trick to work on richer sounding notes. You can to this exercise on any note above probaby A above the staff but Front High F is the easist. Play High F using the Front fingering and bend the note down as low as you can without it cracking. Then Bend it back. It not the most plesant sound to listen to, but it is huge in tone development. I think it really help give you tonal ear focus and helps in breathing. It tends to open your throat up and teach your muscles that all the notes should have a nice open throat. Not just the low notes.

The min/ max really depends on you. The best idea would be to set goals for yourself. If you are interested in making some of the mid-state or all state bands then you would have to put in more time, or if you just want to be the 1st chair player at your school. If you can't sit down for 1-2 hours a day then at least do a small practice, maybe with some scales just so you are not missing a day of practice.