I've had mine on for about a month now as the temperatures had dropped significantly and I was getting my summers refurbed.
The weather has changed so much now,that I will probably change back to my summers to scrub the new rear tyres in.
Because of the softer compounds used in cold weather tyres,you can't really compare them with summers-they do different jobs,so you have to drive and make allowances, accordingly.
Last year,I didn't bother changing from summers and just drove to the conditions, that weren't that bad.
If the weather is like it is where I am-11 degrees and damp-I would thing about changing back,if you want to hoon about-its only a quick job to change back when the temperatures really drop.
It's when the snow comes,that you'll be glad you changed over!