For me it's road conditions more than the temp itself. I don't have the clothing for riding in much less than 0 celcius. Did 10k for fun on new year's eve and my toes and nose were pretty darn cold by the end. It was -5 I think. Other than that, I haven't been riding outdoors since early November because there's too much ice, snow, and gravel on the roads.

Upper limits - I don't love riding in over 35 (about 100 or so) but I do it. I especiall hate hills in that temp because I am working so hard and going so slow, there's no wind to cool me off. But I do it because I live where I do.

Ideal day is 25 degrees, sunny, no wind.