For D&D and superhero games, we seem to get about 1 per 3 hours of play time, unless we're using a published D&D module...the average for those is closer to 1 per hour or even higher. Some Feng Shui games are nothing
but combat encounters, with a few minutes scattered here and there for in-character plan-making and funny quips.
For most of our non-D&D games, we average about 1 combat encounter in four to eight
sessions (something like 1 per 15-30 hours of play).
Not surprising, really; our D&D games tend to be adventure-focused, while the other games we play are heavier on intrigue, investigation, and social interactions. (Plus, many of the systems we use for those games have really atrocious combat systems, which tends to make us all unwilling to waste time with it.)
So basically, we allocate combat encounters in approximate proportion to how good the system we're using is at handling them. If getting into fights in the system is fun, we'll play a game where the characters get into fights. If the combat system blows, we'll make a bunch of pacifists.
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not exactly the most startling conclusion, i suppose
ryan