Our first trip was 9 days/8 nights. We just booked a second trip for 10 nights.
I don't think this represents the "average" trip length, but it just makes sense for us. We keep our food costs to a minimum and the tickets are about the same price as for a trip half that length, so the price of the hotel is the only real price difference. Both trips will be at a value resort, so for about $500 we can have a longer, relaxing vacation. The length also allowed us to do what we wanted, go at whatever pace was good for the kids, take lots of naps, etc. If we were there for only 4 or 5 days, we would have felt that we needed to be in the parks as much as possible to "get our money's worth".