Below is a diagram of the mares estrous cycle.  It is a good example, however there are variables.  I have had mares in for serving that cycles have lasted from between three and twelve days.  The mare is the best indicator of when she is receptive.  I am yet to handle a mare that will let a stallion cover her if not 'in heat'.  Once again I am sure there are mares out there that break the rules! Happy to say we have a one hundred percent conception rate at Bonnie Brae