We let our flock free range in the backyard (supervised) frequently so we keep their wings clipped. Its a personal preference obviously. We are introducing some youngsters to the flock this year and will be eventually clipping them as well for the same reason, so I've recently been asking the same question. Most common answer Ive gotten so far is around 5-6 months. I'm told it is somewhat pointless to do before then because they are growing & replacing feathers so fast that the clipped flight feathers will keep growing back in.
And for what its worth, there are several videos on Youtube that show how to clip feathers. We had our vet show us first hand on one of ours just so we felt more confident, but the videos are very helpful as well.