as long as it's above freezing I don't mind getting out for an hour or two, much longer than that and it's a little chilly. I have ridden down into the teens before, I think the lowest I've ridden in is 11 degrees which was just too dam cold but I didn't have a car and had to work so there I was.
On my Trophy I got caught in the snow a couple of times when commuting. The morning light rain and temps in the low 40's turned into wet snow and temps in the low 30's for the ride home. Not much fun and not real safe either, I'm more cautious now specially with the unfaired Bonnie. My fingers get cold easier now so my rule of thumb is "above 40 and not raining" .
The biggest danger here is Black Ice, usually if it's below freezing it's clear, that means it'll be foggy for a few hours before the sun comes up, the moisture in the air freezes on the roadway and turns the road into a skating rink for a couple of hours.....I don't even like to drive in it, let alone ride.