Going by our group, combat seems to take roughly one round per opponent, give or take how badly our elf archer unleashes Ulric's Fury (seriously, this guy rolls 10's on damage so often that he cranks out over 15 damage per shot).
Part of how long a fight takes depends on how well the PCs are rolling and how good they are at combat. A bunch of 1st career PCs with a WS or BS in the low 30's is going to have a much rougher time of it than a bunch of 3rd careers with scores in the low 50's and up. If you've got a couple of PCs that are geared towards dishing out lots of damage (especially with Impact weapons) and hitting reliably*, then combats can seem brutally quick for the bad guys. We've had some fights that dragged out over a dozen rounds, due both to quantity of baddies (in this case, a town full of Khorne cultists whose existence we uncovered) as or against some exceptionally tough opponents (such as a Troll that was part of an Boss Orc's Warband; sadly for the GM the Boss Orc got TKO'd very quickly).
*It can be done in WFRP, as I've got a Knight of the Realm that's exceptionally good at melee fighting thanks to some very lucky character creation rolls, and we had a Dwarf Troll Slayer that was a friggin' murder machine with that great-axe of his. Also, see Elf Sniper of Doom above.