Too little! The first play-test session was ~6h, the first real campaign (KotS) was ~8 hours.Rather a simple question: How much 4e have you played for far?
For the purpose of this question, we will exclude any playtesting or online play. Sorry, but the times don't correllate well.
Cheers!
I've discovered that combats are becoming shorter as the players get more used to their abilities.
I actually noticed this too. The first battle took about 5 minutes for each turn when everyone checked the book to see how did this and that work again. By the end of the fourth battle it took less than minute for everyone to take the actions.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.