For what species?

Dogs usually come into heat about every 6 months.

Cats are more variable. I've seen them come into heat while they're still nursing a litter, but they can also come into heat any where from a few weeks after weaning a litter to a few months. They are very capable of having 4 or more litters a year.

 

For my Filas, the mother dogs return to heat when their puppies are in their fourth month. Which is about six months from last breeding.
Unbred, or maiden Filas can have quite variable heat cycles, as long as eight months apart. And Filas first esterous can be anywhere from eight to twelve months of age.
For all dog breeds, it's best to wait for at least their second heat to breed. And only to breed her once per year. It is very hard on them, witness the hair and weight loss from the burden of carrying and nurseing the litter, no matter how very well they are fed.