We recommend that winter / snow tires be replaced when they reach approximately 6/32" of remaining tread depth. Tires need more tread depth in wintry conditions to compress snow in their grooves and release it as they roll. If there isn't sufficient tread depth, the "bites" of snow that can be processed on each tire revolution will be reduced to "nibbles," and the vehicle's traction and mobility in snow will be reduced.
6/32" vs Full Tread Depth
Many snow tires feature Winter Wear Indicators (also called Snow Platforms) to identify that a tire's tread depth remains suitable for beneficial snow performance. While Winter Wear Indicators are not intended to be a sign the tire is legally worn out, experience shows that the available snow traction has passed the point of diminishing returns when the tread has worn down to approximately 6/32" of remaining tread depth and the Winter Wear Indicators are seen connecting adjacent tread blocks.
If you are at this point, it would be a good time to start looking for your next set.