I've noticed, depending on the brush you use, they can last a lot longer with the same payoff.
I see a lot of girls using their powder and blush brushes (116, 129, 134, 150) and buffer brushes (181, 182, kabuki) and they're completely WASTING their MSFs!!!
In general, after my powder, I'll use either my Duo-fibre brush (187) or my Contour brush (168) and get the same amount of shine and payoff, but all of my MSFs still have nearly full-height on their domes, despite using them EVERY SINGLE DAY for a full year (some more).
The few that I have in my kit get used almost daily as well, and despite sanitizing daily, they have just a smidge less on their domes as well. I attribute this to the type of brush because of the amount it picks up - there's no mess, either... the brushes are so soft and staticy that the powder isn't brushed off or flies all over the place, it stays in the bristles until it's brushed over the skin.
It made a HUGE difference in my usage when I switched over, with NO difference in payoff. At this rate, my MSFs will last FOREVER. I've even started selling off some of my backups because I KNOW I'll never hit pan on them.