I am assuming you are asking about when in the heat cycle she can be breed. But just in case you are asking about what age she can be breed I'll answer both questions for you.
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.
Male dogs can breed as young as 6-7 months though it is recommended that you wait until they are mature in mind and body which at a minimum I would say is 18-24 months old.
Female Dogs go into heat at any time between 6-12 months usually. Again, 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 as well for females.
You may find this site helpful if you are considering breeding
As to when in the heat cycle is the best time to breed you need to understand a dog's heat cycle. 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.
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. 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. This should help you determine when in her heat she will accept a male.