Well, one of my most fulfilling practice sessions was 2.5 hrs between the times of 10 PM and 1 AM on a cup of coffee, but I wouldn't do this on a regular basis. Unfortunately, I find it difficult to practice well unless I practice for a long time; I guess it just takes me some time to get into it. But I don't usually have large blocks of time available on weekdays (mostly because I waste too much time - I really need to work on that), and I get self conscious practicing for too much time on weekends, because the piano is in the family room, where my dad usually works. So basically, I think the ideal practice environment is when I'm alone and relaxed. I also have enjoyed the few morning practice sessions I have had (I can't do this often because I don't want to wake people up, but I hate the practice pedal).