From zero to two. I try to do as much as possible in school, on breaks and even go to school earlier in the morning so I do not lose a valuable amount of time by context switching back and forth.
Hobbies... I just don't want to do them. Time feels scarce and I fear no matter what project I start, there might be a better idea in my mind (and I have a few) that would be better worth doing.
Write your ideas down, keep a journal[1], but don't let it paralyze you from doing things. Doing fun side things will make those written ideas better and often will gel into something else.
1) I use an actual paper journal. I'm 43 and the idea behind all the stuff I've written finally became obvious this year (while I was on vacation and not thinking about it). Do wish I had started on the whole journal thing earlier.
Hobbies are important, keep them even if the them changes
How many hours a night are you doing homework?