Depends on your skill level and what you consider fun. For most average pilots the fun factor rapidly dimishes after about 15MPH. If Im practicing pattern I will fly in up to 25MPH wind if its stright down the runway. IF its really bad crosswind I dont get much value out of it so I stay home and preserve my precious airplane. With my big gas planes... 20-25MPH. WIth a foamy or a profile plane or something... the sky is the limit[8D]

IF you like flying in a 40 MPH wond just because you can then go for it...