*sigh* mares in heat are the bane of my existence right now...they don't care a whit about my training schedule
Mares cycle roughly once a month (its actually a little less than a month) and stay in heat for anywhere from 3 to 7 or 8 days. Last from late spring to early autumn. If it becomes a big issue, you could look in to something like regumate, though that has its own effects. My friend's mare gets super lazy on regumate and it becomes almost impossible for her to lose weight. But for some mares that have extremely behavior changes and are on a show schedule, its not uncommon.