Five. It follows roughly a Boltzmann-distribution, where the maximum is at around 4-5. It is our statistics from around 200 combats.
(This how a Boltzmann-distribution looks. But in our case, the maximum is at around 4-5, not at 1, as on this picture.)
About the methodology:
Typically I don't write my actual stats in my character sheet, instead we have a table where the stats (mainly, the HPs) are recorded after every round. I used these tables. I have also some other surprising results (for example, spellcasters lose HP with around a similar mean rate as the fighters, but much more unpredictable). There is also a lot of encounters which is not a regular combat, although it uses combat stats (for example, assassin kills somebody in a single round), these weren't counted.
Warning: in our experience, high-level combats last much shorter. These stat is coming mainly from low-level parties. To produce a more clear statistics, at least some of them should be balanced and normalized by the party level, this was not done.