How long do winter tires last?

[Mapman]

I just swapped my wheels today (winters off & summers on for you hemispherically challenged individuals), and after four full seasons of use, maybe 20k miles total, the winter tires still seem brand new, with lots of tread left. One tire has developed a nice bulge in the sidewall, and I'll probably be replacing that one, but like I said, the tread on all four tires shows very little wear. The tires are Michelin Pilot Alpins in 225/40HR17, and have done pretty well so far, though this year they felt like they may have lost a little grip in the sloppy stuff.

So - how long are winter tires supposed to last? :dunno:

[Gary@Tirerack]

20K-25K would be a good average. After they are below 6/32nds they generally shouldn't be entering another season of winter use.