I have been out in -9F before but I had to dress in my snowmobile clothing and boots and could not do more than a few miles dressed like that but it was just as enjoyable, I took my time it was fine. So its never too cold but I do have my personal preference where I draw the line. With my present cycling clothing I do not like to go out in anything less than around 10F. When it gets into the single numbers, its really too cold for cycling shoes, tights and regular cycling outwear. Ski or Snowmobile clothing is required then and that limits your ability to pedal efficiently and cover some ground so its no longer a recreational activity. But if you're commuting to work, running utilitarian errands or are car free, then dress up and go no problem.