I prefer 5 to 10 mph, will take more if it's not gusty or changable. Dead calm makes take-offs and landings harder. I have flown a full-symmetrical wing in 25 mph but can't say it was easy or relaxing.

Depends a LOT on the model I have at the field at the time. My Hot Stik is much more wind sensitive than any of my other models. It gets tossed around like a leaf when I slow down for a landing.

With much dihedral or a flat-bottomed wing it adds some complication. 12 to 15 mph would be about as much as a trainer should attempt under fledgeling thumbs.