To add to what @halo said, most farms (at least in Kentucky) keep to a schedule of 3 breeding sessions a day (10 am, 2 pm, 6 pm). Not every stallion is bred at every session, however, or even every day. It depends how many mares are booked–and even that changes from day to day. And yes, breeding sheds are open 7 days a week from mid-Feb through early June.
We have a mare who is open this year, which means she was ready to breed this month. She started to build a follicle, so she was put in the stallion’s book on Wednesday. Now she’s on the farm’s radar–along with a number of other mares for the same stallion–and the juggling begins. On Thursday, she was progressing as we wanted her to, so we asked for a Saturday morning breeding. It wasn’t available. We were offered Saturday night. We took that, but hoped for something better because mares seldom do what you think they will. Our mare might “slow down”. Or another Saturday mare might come off the book altogether. Which is what happened. We ended up with the Sat. afternoon breeding and the stallion had the night off.
The opposite experience: one year one of our mares was ready to be bred in mid-June. We called to put her on the book. The booker at the farm said, “Other than your mare, he’s done for the year. Just call us when you’re on the way and we’ll pull him out of the field.”
Most of the popular stallions in KY are bred 2-3 times a day from mid Feb through April. Then things begin to slack off in May and June. A few stallions breed more often than that. The less popular horses breed less often.