Not sure how some of you guys get that sort of mileage out of snow tires. I typically put them on mid November and take them off late April. After 7-8000miles over the winter I find blizzaks are typically half worn on the q7. I have seen the same results with dmv1, dmz3 and LM 25s. YMMV
Blizzaks have a notoriously short effective life and turn into all-seasons after one winter of good use, that's why I've never used them. I get 3-4 seasons from my Michelins (deeper siping and winter compound depth) and the tread life is warranted for 60,000 km. Of course a lot of it comes down to the climate where you're driving. If you live somewhere where the temp is at the freezing point or below for the entire season, then the tread will last approximately as long as a summer tire because the soft compound will harden to it's optimum firmness in cold temps. If you live somewhere where it's above freezing during the winter and you only get into cold temps during occasional forays into the mountains or brief cold snaps, then your tread life will be drastically shorter because the soft compound won't firm up.