Generally, quite a lot, as long as you stay dry. I spent 8 straight hours outside shooting pictures while standing - and occasionally lying down - on a frozen lake in northern Canada in January. It was -25C / -13F. I was perfectly comfortable, although I did have to repeatedly switch batteries with ones that I'd been keeping inside my coat.

Goal was to get pictures like this: https://goo.gl/images/imXCMy

(Those are methane bubbles frozen in the ice.)

"It's not the [lack of] heat, it's the humidity" applies in the cold, too :)

It's occasionally getting to -20-25 C during winters where I live, but people go around in their daily routines (only exception is children that do not have to go to school if it's below -20 C). I'm pretty sure if not the clothes, there would be a hell lots of frozen bodies around every winter.

Clothes are the reason people don't regularly die when they go skiing, or for that matter, live outside the tropics.