Its going to be a measurement based on how much force was used in pulling it. Looking at those bands, the pulling of one band has to use up its force pulling the next band. Energy is lost in this system from friction of the rubber against the rod (minimized with lubricant) and the movement of the air as a result of the fan spinning. As well, some heat is lost heating up the rubber (the side effect of the tension of the rubber changing). Some is also lost in friction of the fan shaft.

I posit that the fan (if there is no inbound air force) would likely spin longer without the rubber bands given the same amount of input rotational force.