If a mare waits more than 12 days before ovulating during her foal heat, she's perfectly breedable that first heat. If you don't breed her, and don't short cycle her, she'll have approximately 14 days of diestrus, then will begin to come back in again. So her first post-foal heat heat should happen somewhere between days 26-30 post foaling (If I did all that math correctly...)
If the mare waits before ovulating her foal heat, that heat is breedable and just as fertile as a normal cycle. After the foal heat, the cycles are completely normal and should be as fertile as that mare is normally. If she ovulates too soon during her foal heat, that foal heat will not be fertile and you should wait for her next heat.