Kris, it would be impossible to give you a number. It depends on too many factors, air temp, load on the bike, riding conditions, How fast you were riding just prior to hitting a traffic jam, etc.
Suffice to say in almost daily commuting last summer my fan came on only once. It was after about 10 minutes of stop and go following 20 miles of 75mph in temperatures near 90 degrees F. as I waited for an accident on the South Grand Island bridge to be cleared.