120k Miles Original Fluid topped off once. BMW says don't change the fluid the risk of contamination is greater then the benefit of changing it, just keep it topped off with the right fluid. (This is for Non-M although we share the same fluid.)
And its not exactly boiling, the PS cap vents, 9/10 cars Ive seen have fluid venting out of the cap.
You also don't have to buy it at the dealer, a few local auto stores carry it here in Socal.