In a 4 hr demo with 6 players, I explained the rules, we did a bit of roleplaying here and there, and completed 4 combat encounters. The first encounter was about 40 minutes, the second encounter was a difficult one and took about 1:30 minutes, the other two encounters were about 20 minutes and 40 minutes. Didn't time any of them, these are just estimates, based on the 4 hr time frame.
I figure each player needs around 30 seconds to take their turn (some will be longer, some shorter). The DM takes about 3 minutes to take his turn earlier in a combat, around 1 minute late in the combat, say average 2 minutes, add another minute for gory descriptions, that's average 3 minutes. 1 round of combat should be about 6 minutes. Add another minute for interrupts, recording values, and such. 7 Minutes. An average combat of equal level should not take longer than 5 rounds, which is 35 minutes. A Level + 2 encounter may take around 8-10 rounds, with more dailies and monster activations lengthening combat, so it can take around 1:15 minutes. Add a bit more time to that for a Level+3 or Level+4 combat, so I would plan for at least 1:30 for those challenging encounters.