Like Eden, I don't really have a limit unless it is both raining and cold (below 35). We did a 200k this December where the average temp was 41 and it rained for the latter half of the ride (we really thought/hoped it wouldn't rain). That was more miserable than the ride I did a couple weeks later where it was below freezing (but dry) for 3 hours - warming to the late 40s later on.