To me one of the most important things is to keep my love for the game irreducible. I have quitted the "make it as a pro" dream long now and do not try to force myself into playing x hours a day, I play for as long as I am happy. Instead of forcing myself to play another 500 hands, I often enjoy reading a pkr book, posting a hand, watching a training vid or reviewing my notes/tracking my progress.
I just play until I'm no longer having fun, which is usually 2-3 hours, after which I'll take a short break maybe 15-30 mins and then decide whether I want to continue playing or not. I think my longest session was somewhere around 14 hours, but that was back before I really knew what I was doing. These days I almost always do 2-3 hours then I sit and evaluate my mindset and gameplay, and how good the table selection is, etc.