The time to mount winter wheels is when the average daily temp drops below 7 degrees Celsius. As Narvselius points out, for eraly morning drivers, that time has arrived in most of Europe now.

I have mounted my winter wheels a few weeks early and removed them 1-2 months too late on my E61 for the past few years, and haven't notices any excess tyre wear. Of course drag traffic light starts on warm days should be avoided.