I believe it's twice a year, like every 6 months and it lasts a couple weeks. We got my beagle Loki when she was 5 months from a family that couldn't keep her any more. They hadn't had her spayed yet. Well right after we took her home, within a week or so (before we could get her in to get spayed ourselves) it started! She had to wear little denim panties for a couple weeks LOL