Depending on vet/horse/work load in my experience 6 months to a year is normal, but 3 months seems to be the cut off point I hear. As if if the horse is showing lameness/symptoms something needs to change, usually a decrease in work load. Of course this can vary because vets have different ideas.
Completely arbitrary rule of thumb... but when I was giving Stoney hock injections yearly and 2x/year, I felt like that was completely average. When I started to think about them before the 6 month mark was up, I started to feel a bit uncomfortable. Once it got to the point where it was more than 3x/year, that I wasn't comfortable with for an older horse that owed NOTHING to anyone. Turned him out in retirement, didn't ride for a couple years. Hopped on a few months back on a whim-- SOUND. I think the hocks finally fused once I left them alone! He feels GREAT now. Go figure!
My older jumper has been getting both coffins and hocks done every six months for years. When I was starting to feel like he needed them more often than that we added Tildren to his maintenance schedule. Since doing that I've been able to extend the time in between the injections by a few months. His hocks also respond well to shockwave and pentosan. Injections are not your only option these days!