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.)