I agree with iddee, that 45F is a good cutoff for safe flying vs not flying.
I’ve seen some of my bees flying at 32F/0C when it’s sunny. That is one of my concerns about getting the winter hives TOO warm. If it’s 70F to 90F inside a hive in the middle of winter and the bees see sun, SOME of them will fly to check for foraging. At 32F, you’re going to lose most of those bees. My bees will make cleansing flights in the winter if it sunny and the mid 40s. You don’t lose too many bees on the cleansing flights because they just fly out, do their business, and fly back in. This is an advantage of poly hives in the winter, the bees are warm enough to really become active (cleansing flights) during a warm spell. Here in Michigan, a warm spell in winter is the 40s.
I’ve seen bees milling about on the landing board when the temperatures were in the teens. That was in super insulated polystyrene hives.