I read the instructions and they suggest leaving it on for an hour!

My mare has become extremely itchy this year her back legs are the worst! I did try Benadryl and it helped a little, also just bathed her in a medicated shampoo.
Vet was out and gave us non-steroid men's, hopefully, they help. She did say this year she is treating this a lot.
Any other suggestions to relieve the itching?