Well, it's not referred to as a "period" in dogs. Dogs "come into heat" are "in season" or "in estrus". Unlike in humans, this is when she can become pregnant and the only time she is interested in mating (and males are interested in her). Here is some info about the reproductive cycle in dogs:
vetmedicine.about.com
www.askabreeder.com

The age a female first comes into estrus is variable. Can be as early as 6 months, as late as 2 years. (Though that's not common.) It happens roughly twice a year until she is spayed or dies.

Bruno CGC answered on 7/6/12. Helpful? / 0