Wind is purely personal preference and experience. I've flown .15 powered combat in winds that were clocked at the local airport as gusting over 45mph (about 72kmh?). But that was a specific case of a contest with airplanes that can handle it and experienced pilots. You won't see me fly in that wind with my Cap or a trainer.

I have trained students on a buddy box in winds that were rather strong though. I took the students up high and had them practice slow flight handling by parking their airplanes in one spot over the field, or even getting them to move backwards slightly. (and, wouldn't you know it, the guys who did this started landing better when they were out later in better conditions to actually try to land). I'd say the wind was in the 15-20mph range. But I also didn't let any of them near the ground in those conditions, even students who had landed a few times before. The turbulance close to the ground made it hard enough on me to land.

Wind, IMHO is like any other situation. It's a judgement call. Part of learning to fly is learning what you are comfortable with, and learning to apply good judgement. There is something to be said for being careful.