Every year is a bit different. Some years I've put her away as early as the beginning of November, others as late as New Years. It has nothing to do with the cold (have been seen driving with full winter parka, hat and gloves) but, rather, road salt. In fact, I think they've switched over from salt to some other chemical treatment in recent years. It appears more effective, washes off the roads quicker than salt but, man is it corrosive. It really rips the roads to shreds and I've actually seen it pit chrome on good cars.
One nice thing about putting your car in winter storage is that when you finally pull the cover off and fire her up in the spring, the excitement and anticipation are almost as good as when you first bought the car or went on that first drive after a resto - what a blast!
Bob
PS: some day if I have more space and some heat in my garage I can schedule my project work in the winter
PPS: then again, I think the wife likes it when I "take a few months" off from my obsession!