It really varies with us too, much like what's already been posted. The type of action the sheets have seen is the absolute deciding factor of course.
Might be washing sheets, and especially the king sized mattress pads, more often now that the Affinities have arrived. At least until the novelty has worn off, then it will be back to the "hey, how long have these sheets been on here?" routine. Sad commentary when the "action" reason isn't as much of a factor as it used to be . . .