For me it depends upon the model I plan to fly. Some of the faster models that are still forgiving don't mind the wind so much, but the warbirds are too precious to risk in a wind (really don't fancy the wind suddenly dropping when I was depending on 10-15mph from it on approach) and the sunday flyers have too high a gust response and it makes flying no fun.

In general, I usually draw the line at about 15-20mph if it's steady and 10mph if it's not. I have flown in stronger winds, but to my mind at least, it's no fun. Sometimes I will fly if I have a model that can hack it, if I arrive at the field and it's stronger than expected.

When I'm with a learner (and in fact when I was learning) I'd prefer to keep it to a steady 10-15mph or less if possible.

For me, the primary concern is the airspeed when low & slow as I noted above, but for begineers especially, a strong wind is quite unhelpful for circuits too, as the upwind takes forever and the downwind leg takes seconds. A strong wind will carry a model a long way in a short time which adds to the workload of any pilot, but for a beginner it's possibly one thing too many???

In winter time, when there's no leaves on the trees, I only go flying if the branches aren't really moving. In summer it's harder, but I don't really want to see much more than the leaves rustling. The branches will move at slower wind speeds with leaves on, but you don't want to see the overall "tree" swaying. Just my rule-of-thumb windsock when looking out the window wondering