Is this mare going to be okay for you to ride, considering the health condition you have mentioned before? I own a six year old mare myself, so that just concerned me a little.
My mare is in heat every other week it seems. I live in California though and we aren't having much of a winter so far (again), so it's pretty normal for mares here to cycle all year long and fairly frequently. The other mares at my barn are the same. Mine will come into heat for only two or three days and then go back out, probably every 8-10 days or so. Another mare out there seems to be one week on, one week off. Then there are two Saddlebreds who never seem to come out of heat. It really just varies from horse to horse, probably based on a number of factors.