How long is your average set

I never ride with serious riders so there are a few things that I just don't know what is the standard or normal. How long is (as in time) is your average set? Total time including getting your board on and time spent in the water between falls and restarts.

I guess I'm curious for a number of reasons. I've heard people say they kick in x-number of dollars per set they ride, that they might get in a certain number of sets in an outing, or that my boat burns x-amount of gas per trip if 6 riders go out and ride 3 sets each. All of which would mean more to me if I knew how long a set is. Personally I find that my average set only lasts between 10 and 15 minutes before I'm feeling tired and getting sloppy but I'm not riding regularly and feel like my arms and legs are under-conditioned, I never really thought about it till the other day when I was paying close attention to my watch. Most of my friends that come out riding with me are about as experienced as me or less and generally only ride about 10 minutes.