Since not all dogs can be created equally, because of their size, breed or life conditions, this question will be best answered depending on the average dog.
Female dogs typically begin going into heat, which is also known as the estrous cycle, at about 6 months of age. It is not uncommon for females to experience their heat as early as four months, or as late as fourteen months. However, if your dog has not gone into heat by the age of fourteen months, a visit to the veterinarian is recommended.
Once your dog has experienced their first heat, you can normally expect a heat once every six months. It is also not uncommon for some females to only go into heat once a year. In the instance where you have two females living in close proximity, they may begin to have their heats at the same time as each other. This may have one or both of the females, going into heat earlier or later than expected.
The best way to know if your female dog is going into heat would be to know the symptoms. Swelling of the vulva, a light bleeding, and possibly increased urination are good signs that the heat cycle has started. The best indication would be that males will be come increasingly interested in her.
If you don't want your dog to breed, keep her under constant surveillance and on a leash when she goes outside. Consider having your dog spayed to avoid another heat cycle or pregnancy.