Is "all of the time" a good enough answer for you? Because that's when they usually run at GenCon. All of the time.
I know there's a lot of games running from 9am-1pm, 1pm-5pm, 5pm-9pm, and 9pm-1am. Four hour blocks seem to be popular, but most pick up games last a little less than that. Between 1am and 5am is a short lull in the gaming action while less hardcore gamers sleep. Psht.
If you could be a bit more specific as to which time of day you're looking to run, and on which day, I'm sure I could help a bit more directly.
-TRRW