If it is "by the book" a mare's total cycle is 21 days. She should be in heat 7 days, out for 14, then back in. So count 14 days from her last day of heat and that's when she should be back in. Of course we all know horses, sespecially minis don't always go by the book, so I start teasing at 12 days from her last day of heat just in case she showed signs of being heat another day or so after ovulating, or if she short cycles herself.