I'm probably 4 hours at the moment.. but when I was not working that number was easily 10+ average and for the record... way back when 1.8 came out and I was in mapping/modding prime, I literally got up at midday and was on the PC until 6am every single day.. with various foods, drinks and substances to help me
I reckon I did 8 months of that... and it was FOOKIN AWESOME!