about 8-10 hours, my job is 8-9 hours and I jam some Batman or Bioshock 2 (or whatever game I have installed and are busy with) when I get home some nights....
On a holiday at my dad's, i get on about 11 when he goes to bed, and stay on all the way through the night. Should be on more though, i've got boxing and football training. Then i need to go out to play as well. One year i'm just gonna play FM non-stop, for a full year!