Everyone's limit is different, and conditioning and skill are big factors.

Last winter, my limit was about 15ºF. My commute is 14 miles long, and it takes over an hour. Exposure that long gets painful. This winter, I bought better clothes, so I hope to increase my tolerance for hour-long exposure. NYC is a very windy city, and my route is right next to the Hudson River, so whatever the weather department reports as the temperature and wind speed is not true for me.

My only incentives for pushing my limit are to have fun and to feel badass. I really don't need to do it. My other way of commuting is to walk 15 minutes to the subway, ride the subway, and then walk 5 minutes to work. And the subway works almost perfectly in bad weather, so it's really a very sensible way of travel in bad weather. But reading all these super badass Canadians and Minnesotans here about how they ride in -40º weather makes me want to push my own limit.