It really depends, though I’m trying to get up to 4-6 hours per day. That’s my goal, anyway. I did slightly over 3 hours yesterday.
I should note that I count just about anything musical as practice, and not all necessarily sitting at the piano or even having anything to do with the piano. I count rehearsing for choir as practice, too, and practice with my piano ensemble group, etc.
I don’t really think there is any such thing as too much time, if you practice properly. I practice in sessions of 20 minutes on a single focus, and then start something else. Every hour, I take a 10 minute break. I’ll usually go out and sit in the lounge and try to socialize.
I split all my pieces up into sections that I want to work on for these 20 minute sessions. They might be sections requiring technical work, phrasing, better interpretation, musicality, etc. The concerto I’m working on is split into about 45 sections (this is the first movement alone).
So usually I’ll have a practice schedule prepared for the week, of which piece I will work on for each 20-minute session. Then I just have to choose which section of the piece I want to work on (I haven’t included the sections in the schedule yet).
I don’t work on a section more than once a day, though. I’ll work on the piece still, but just on other sections. I find this helps to keep things fresh and prevents from getting too repetitive.
I did that last week for this section with very fast alternating octaves in both hands, and I was so much better at it this week, so I think I will continue this method.
The trick is to prioritize sections, I think, because obviously I can’t get all 45 done in a single day. I’ll have to work on, say, 8 for a week or so, then swap out a few of them for other sections.
Anyway, that’s what I’m doing now. I change my method every so often to see what works, but this has been the most promising so far.
I do have to comment to those who say that one should only practice when one feels like it. If you only do this for enjoyment, then that’s fine, but if you’re a music major, you’d have a rather angry piano teacher if you did that. With so many classes and also the need to study, you have to fit it in whenever and however you can, regardless of how you feel. I’ve found that once I’m actually at the piano, however much I didn’t want to practice before, I start to get into it and enjoy it. I basically stay out from about 8:30 in the morning until 10:00 at night (minimum), and practice every chance I get.