For me seems like 10-15 mph is ideal.  20 mph is OK as long it is consistent and in a good direction.  Once it gets to around 25+ mph I usually pack it up.  It stops being fun for me when I am doing alot of correcting and saving of the plane.  Sort of like the plane is flying me instead of me flying the plane.  I also usally take my GP Big Stik 40 out when the wind is really blowing.