It's 7 degrees, according to a duck.. sorry, ADAC ect. This is a bit of a piece of string question - if you live out in the wilds and you need to get to work early in the morning then yes, as soon as. If you only do journeys that can be differed 'til later then meh, as and when. Here in Croatia we have to have winter tyres from November 15th 'til April 15th, no matter how stupidly warm it gets in March.