About 20 mph for me, I would push that to 25 mph if the weather had been windy for a week or more and I hadn't been out much as a result.
I prefer light winds of below 10 mph, but usually find they are between 10 and 15. I try to plan my route so that the wind is behind me on the way home, but living on the west coast, that is difficult when the wind is a westerly (although a mate of mine who works on an Irish Sea oil rig did get a bike on the cycle to work scheme, so you never know

). I have managed to develop a few routes that minimise the time you face a headwind for winds of different directions, but sometimes I think if we go and find hills to climb to keep up our "hill skill", why do we try to avoid headwinds so much, they give us a great workout just like hills?