I'd say 2 to 3 hours. But then, you asked for an ideal duration.

In reality, even a 100 points Infinity skirmish usually takes well over 3 hours with me and my friends, as we are usually chatting away during the game, seriously hampering the speed of the game. It does not even matter which game we play; it always seems to be dragged out due to the good times being had. Heck, even a game of Munchkin takes us an entire evening!
And that's the whole reason we play the games anyway; to enjoy ourselves. And you can't put a time on good times!

Then again, we've all played our shares of tournaments, and on the whole, we then managed to play our games within the allotted timeframes (with the odd exception here and there of course

).
Still, all things considered, my ideal time would be about 2 hours per game, so that in an ideal world, it would be (theoretically) possible to have two games in one evening. But leave it to chatty us to not make that a reality. Ever...
