I pair for 3 days at a time or until they separate. After a successful lock, my female gets at least a week off before the other male goes in. I'm rotating two males in with her, so they get a total of about 6 days a month each.
Just like in human reproduction, one lock is all it takes, but it's all about timing. If you've witnessed locks but the female hasn't developed follicles or ovulated, it probably hasn't gotten the job done. This is my first year breeding, but I've been told to keep pairing them until they stop locking.