From Tirerack:
When are winter / snow tires no longer appropriate for the next season?
Check your tires' tread depths. When a winter / snow tire has worn to about 6/32", its ability to provide beneficial snow traction will start to diminish. While still legal at 2/32", a winter / snow tire has worn well past the depth where it provides beneficial snow traction.
Also read: Tread Depth – Why Too Little is Never Enough
http://www.tirerack.com/tires/tirete...sp?techid=163&
-------------------
How Long Will Blizzak Winter / Snow Tires Last?
The Blizzak WS-tires’ Multicell tread compound comprises the top 55 percent of the tires' tread depth with a standard winter tread compound comprising the remaining 45 percent. When Blizzaks are approximately 50 percent worn, their "snow platform" tread depth indicators "connect" selected shoulder tread blocks to indicate that the Multicell tread compound is almost worn out, and that the enhanced snow and ice performance is about to end.
We have personal experience on a wide range of vehicles from sports cars to SUVs. Our experience, and that of our customers, indicates that Blizzak winter / snow tires will typically provide 12,000 to 15,000 miles of winter service before wearing out their Multicell tread compound. Therefore, Blizzaks are providing an estimated three to four winter seasons of wear from their Multicell tread compound for the driver who drives an average total of 15,000 miles per year (with 3,000 to 4,000 winter miles per year).
Blizzak winter / snow tires are not recommended to be reinstalled when their tread depth has worn down to the "snow platform" tread depth indicators before the start of the winter season.