It depends really on what I'm playing. I could spend an hour coordinating the bombastic octaves with a completely different melodic line in a Liszt piece, or I could spend an hour working out the voicing in a fugue.
That being said, I usually can't sit for several hours on the same piece (other people are different). I can do an hour, then a break, then another hour later on the same piece. I also don't like doing a bit of one piece, then getting side tracked with another (unless I am simply playing through them). If I'm working in depth on a Bach piece (which I WAS doing today), and then suddenly switch to some Chopin 'for fun', I'll end up mixing up what I was accomplishing with the Bach.
So anyways, I'll usually do 2-3 'sessions' a day, rather than a 6 hour burst. They'll last between 45 minutes and an hour and a half. If I have the whole day to myself, then I'll do 3-4 sessions like that. If I devote one chunk of time at one piece, I find I accomplish WAY more! If I'm sitting for 3 hours rotating over and over between three or four pieces, I just feel really disorganized.
I also have my little creative part to my day (usually late at night), when I'll mix in some technique with some improvisation. I make up a lot of little exercises for myself to work certain things. An example of something I did today was large spanning double notes (to see how far I can actually reach

) So I played 35-24-13-24-35 on EC - CG - GE - CG - EC (intervals of a 6th, 5th, then 6th, then 5th, then 6th) with my right hand, as legato as possible. I did this going down each major key. I spent a good 20 minutes on that, then I practiced some chromatic minor thirds scales (now that I actually understand the fingering) for a little while.