40 hours. Just kidding. 30 minutes, 60 minutes, 2 sessions of 30 minutes, 4 hours - everyone is different.

Figure out what works best for you. I like to prioritize achievement of tasks over how long I spend.

I try to spend 20 to 30 minutes on a problem I'm trying to fix in a piece, before moving on to the next problem in the piece, or the next piece of music.