The campaigns I've been in (3/3.5) tend to have really drawn out battles. With at least 5 players people tend to zone out or have conversations when it's not their turn, so it takes a while to get them back up to speed before they can even decide what they want to do.
But that number only goes up as our level goes up. By the time we finished our last campaign that ended at level 27 it was closer to 95% combat (since combats took so long to finish).