Mares generally ovulate 24 hours prior to going out of heat, so figure from the last breeding and not the 1st.
Big horses can be done as early as 10 days post-ovulation, but not having the uterus in your hand is a disadvantage for minis. I'm more comfortable at >14 days. If you consistently use your Vet for this procedure, her skill and comfort level will improve over time.
Early detection of pregnancy has advantages:
1) No longer need to re-tease a mare which is a dangerous situation for mare, stallion, and handler.
2) Showing signs of heat or not is not the same as being pregnant. Although it is the norm for pregnant mares to not show signs of heat, it is also not 100%. Additionally, just because a mare doesn't show signs of heat doesn't mean she is pregnant 100% of the time.
3) Because mares have a long gestation, quickly returning to pregnancy increases production. Mares that lose just a few weeks each year a few years in a row will eventually have an open year. Open years are very costly. Knowing if a mare is open at 14-18 days allows for short cycling and saving those days.
4) The opportunity to diagnose reproductive issues is between cycles. If we know at 14-18 days that a mare is open then we have a chance to take a culture and cytology which allows us to treat the uterus prior to the next ovulation.
Dr. Taylor