Female Labrador Retrievers go into their first heat between 6 and 24 months of age, which is the same age range that all dogs are given. The Labrador Retriever breed tends to be toward the latter end of this age range due to its larger size.
The dog will then continue to go into heat approximately every 6 months or twice per year. Most female dogs are in heat for 18 days, although male dogs are only interested in the female dog for about half of this time. It is recommended that if the owner does not intend to breed the dog, then she should be spayed either before or after her first heat cycle.