Like TerraDave I find that "combat" (using the definition above) in 3.XE D&D takes around 70% of the time spent playing, but I'd really rather prefer that it took 50-40%.
In most other RPGs it takes 20%, except Exalted, which is up at 70% like D&D, and good old Champions, which was, when we played it, more like 90%.
Still, with D&D's tactical-style combat, 50% sounds like what I'd like.