Of course, now that we're in the era of premades and crossrealm battlegrounds, things can be much, much faster. Here's a video, linked from the thread, of a Horde premade finishing the thing off in just under four minutes.
Unfortunately, not all groups are as well organized (and knowledgeable-- what's that deal they do with the Generals at the end there?) as this one, and I'll often log into a WSG late at night hoping for a quick 15 minute match, only to have it turn into a 45 minute grind as one side hides the flag and/or farms away the HKs. As for AV, my average time is about 45 minutes or so, but never much longer than two hours. What are you seeing out there in the BGs? I'm sure nothing is as fast as they're predicting arena matches to be (under 5 minutes each, we've been told), but we don't still have weekend long AV games. Do we?