Some labradors will enter heat on the clock every six months and continue this way during all of their sexually mature life. But some labradors will see longer Anestrus stages which will completely turn those calculations off the table. Some labs can go as long as eight months between heat cycles, and in some extreme cases, they will only go in heat once every year. Just like with humans, everybody is different, and since the Estrus cycle is much longer than menstruation, it’ll take some time for the peculiarities of your lab to be apparent.