I should time how long the combats in my home game are taking. To tell you the truth, I don't really know how long they're taking, but I know that it seems about right. I voted 45 minutes, just off the top of my head, not backed up by any empirical data.
I've played console games, like Final Fantasy Tactics, etc. where the quick non-boss combats can take 15-30 minutes, and I feel as though that's too short--it makes the combats repetitive, infused with a feeling of deja vu.
If the combats are 45 minutes long, it means that a 4 1/2 hour session with three combats has 2 hours and 15 minutes worth of combat, exactly half the session. If it has 4 combats, or two normal combats and one big combat, that's 3 hours of combat and 1 hour of other stuff. That feels about right to me, but that's all personal preference talking. I like combat, and while I like the other stuff (exploration, NPC interaction, shopping) too much of it in one stretch starts to feel long.
To put it another way, I think probably 50% of my current sessions follow the 4 1/2 hours session, 3 combat model above. Some sessions have more roleplaying and some more action, but I have players that when we spend more than about 45 minutes out of combat, unless it's exceptionally engaging, they ask straight out "are we going to fight something here?"
I have felt some
4e combats getting long, and I have some techniques that I've developed to avoid that. Maybe I should start a new thread on that though.