Once they're in their 20's, I just watch each year to see how they're doing. I've bred them up to age 24 with no side effects at all, and all nice babies. I tried one at 25, but it was hard on her -- after many successful years of breeding, so as long as she's doing well, and having a good and easy time of it, I'd continue, until you see a foal start to pull her down during the lactation period before weaning. As long as she's doing well, I'd just keep going with whatever schedule you have for her.
This DEFINITELY needs to be repeated!!