I look at a wind chill chart to let me know what will happen to my exposed skin. Usually anything below 40 is not going to pleasant for any length of time. In my younger days there was really no limit to how cold I'd ride but time has a way of bringing you to your senses.
My ideal temps are between 40* & 90* for jeans and jacket. Colder than 40* I brake out the heated gloves and thermals. I am behind a batwing and lowers though. I would rather ride in cool weather than sweat my ***** off.
I'll go into the 40s as I can do it comfortably. I've done below 30 which was not fun, but it was a test ride in winter after installing parts, so it had to be done. I am considering investing in some heated gloves this coming winter since my hands getting cold is my biggest concern. When I curtail is once there has been snow and you have the piles of salt or sand on the road, along with all of the gravel debris. January and February were the only months I didn't get out there the past year. Every other month I was able to get out for even a few days.
(without wind chill temps)..... Usually under 35 its Heated gloves, heated vest & lined leather chaps, besides heavy leather jacket, and FF. Have left the house when it was 18 and was warm and toasty.