Current commute (11 mi one way), I'll ride down to about 15 F, that's the point when my feet get too cold despite my best efforts, and it's not cold enough long enough here to buy dedicated winter boots/shoes. It takes some mental fortitude to get out the door when it's 35/raining, but it's worth it to see the co-workers faces when they realize I rode to work I'll take the train if it's too icy, since it won't stay cold long enough to bother with the studs the past couple years...

Old commute (2 mi one way), I rode down to about 5 F, it was short enough that the extremities stayed warm....and we didn't get that many days that cold, so I'd challenge myself to do it

Non-commute riding, probably 35 F if it's going to be sunny and not super windy.

Of course, it's 55 F and foggy this morning, so I was way too hot halfway to work despite losing several items of clothing from the recent commute wardrobe....