Female Dogs go into heat at any time between 6-12 months usually. The average heat cycle for a dog is approximately 3 weeks but for some dogs it is longer and for others it is shorter. Some as short as 10 days and some as long as 4 weeks or more. Usually your dog will go into heat every 6-7 months though again each dog is different and can go back into heat as early as 4 months or not for 12 months. Most however are average.
Before breeding your dog you should get the dog health tested for any potential diseases prevalent in the breed. This helps ensure healthy puppies. Different breeds have different health problems associated with them. In many breeds Hip Dysplasia is a problem and is generally tested at 24 months. This helps to be sure you are not breeding a dog with a health problem that potentially could be passed down to the puppies.
You want to wait until they are mature before having to raise pups of their own. Also breeding a younger female can result in additional problem such as smaller litter size and additional health problems in the mother. I recommend 24 months for females. The AKC will not recognize a litter from a female that is under 8 months of age or over 12 years old unless special documentation is provided.
During the first part of a dogs heat period you will notice swelling of the vulva, possibly an increase in urination and bleeding. If there are male dogs around you will notice them hanging around her. This stage generally last about 7 days and she will not allow males to mate with her during this period The second part of the heat cycle is when she will accept a male and breed. Usually bleeding has stopped or become straw colored at this point. This can last from 4 to 21 days though the average is 7 days. It is during this period of time that you will want to allow mating. Most breeders allow the dogs to mate every other day during this period.
Then during the last stage of heat she will be less willing to breed as she is going out of heat. This stage can again last from 4 to 21 days as well but averages 7 days.
Each dog is different so some evaluation by the owner is necessary. The whole heat period should be approximately 21 days but as stated can be longer or shorter depending on the dog.
Here is an excellent website with pictures to illustrate a female in different stages of heat.
I hope you find this information helpful.