^ Someone, on here, mentioned that places like eBay are flooded with part worn winter tyres from Germany, all with about 4mm tread on them. That says to me that the Germans change their winter tyres at about 4mm, and if they do that, that suggests that at 4mm and under, they aren't as effective. Which makes sense since most winter tyres come with 9/10mm of tread, so you're only getting 50-ish % of the grip with 4/5mm ones.

Mine are second hand, because I can't afford new, but have about 7mm on them (been run for only a couple thousand miles). So long as you take them off before the ambient temp gets to 7deg again, then you shouldn't prematurely wear them out (they will wear quicker in the warmer temps (7+deg) and dry spring roads. I'm gonna keep mine on till Feb probably, unless we get a particularly mild winter.