I have limited myself to 20F for commuting or errand running of distances over 3 miles. Because of a flat I had once late at night 5 miles from home, 15F and no one to call. I had a patch kit and a pump, but by the time I got it all together my hands were very near frostbit. I will admit to some rides in lower than 20F down to 0 but I make sure I have tubes and cartriges (a splurge for me, common to others).

So ride in whatever is comfortable, just have a plan for a cold weather flat because that plan may need to be different than your warm season flat kit.