I give 10 days after an accidental breeding and have good luck with that. Like the others said, you could wait a few days and maybe save $3 if she cycles anyway. On the flip side, I'd try to get it done by 30 days. I tried aborting a cow that was 3 months bred and saw no heat. Two weeks later I gave her another dose and still saw nothing so I turned her out with the bull. At preg check time she was still bred to the first mating. The vet said it won't always terminate the pregnancy once you get to 3 months bred or more. I was moving her from fall to spring calving so I left her exposed to the bull and wasn't in a hurry to abort her.