About 5 to 13 hours a week with quite a bit of fluctuation. Majority of my gaming time is actually from Sunday night to Wednesday night. Thursday, Friday and the weekend I am either too busy or away from my desktop (out of town to visit family). I guess I also play about an hour of FIFA 10 on the PS3 every week on the weekends with buddies.
Much of my gaming time is really dependent on the game I am playing and how much it hooks me on. When I used to play L4D, I would be gaming whenever I got the chance. Also, after I have just finished a single player games campaign/story there is a period of a few days or a week in which I do not play any games. It is similar to the empty, sorrow feeling that you get right after you finish a good book; you do not want it to end but you just can't stop reading/playing.
I have a huge backlog of games from steam that I am slowly playing through. The problem with backlog games is that they have already lost their new and fresh status even though you have not even played them yet. You see them everytime you open up steam, and they quickly lose their attractiveness. Another issue with backlog games or just old games with bargain prices in general is that the multiplayer component is no longer existent. WiC, Crysis Wars and CoH are examples of good/great games with fun multiplayer (maybe not crysis) that is really small/ non-existent. Luckily for me, I care a lot more about singleplayer and a good story than multiplayer.