My chiropractic vet prescribed Legend for my horse this year who was showing some discomfort in his stifles at the height of the show season/summer. She indicated that she normally prescribes Legend for horses with stifle symptoms/issues and Adequan for horses with hock symptoms/issues.
Her recommendation was to give it 3 days out from a competition. After using it a few times, I think 2 days (or maybe less) would have probably been better, as a previous poster said. It does work quickly and I could normally feel the effects the very next day.
As far as how long the effects last, I found that I would feel a notable difference in my horse for at least a week. Overall he would feel looser, lighter on his feet and more relaxed throughout his entire body. The greatest improvement would be in the first few days post injection. As the Legend wore off, he would show more tension due to discomfort.
If it works for your horse, the results can be really amazing and worth the price per dose.