Of course it's variable, but when I'm having a good time, I try to make it last. It's normally past the 30 minute mark, and I can do multiple rounds too (usually 3). A couple of particularly good meets lasted well over an hour each round. That said, I normally last less than that when I meet someone for the first time, either because of the novelty excitement if it's a good one, or because I'm not really feeling it and want to finish it.