Personally, after two games, I seem to run out of mental stamina to continue playing. This usually means about an hour.

How often does everyone else play for per session?

(I may end up playing 1~3 sessions a day though. SC2 is fun.)

I get in about three to six games per day currently, which isn't bad. I would like to get more in since I find the 3v3 bracket to be loads of fun. I tend to be more on the competitive side when my buddies are with me. :P