In reply to Goofyfoot:
I don't think there's any definitive, objective answer to that question. It's going to depend on your definition of "winter", how fast you move, how hot you tend to keep, what other layers you want to use it with and myriad other factors.
It's definitely not going to be too hot for stopping for lunch on Ben Nevis in a blizzard. It's probably a bit much for a fast-moving walk around the Peak District at the moment. Anything more useful than those two extremes seems to depend on all the factors I mentioned above.