The average heat cycle for a dog is approximately 3 weeks and since this an an average, some heats are shorter (as little as 7-10 days), others are longer (4 weeks or more). Average times between heat periods is seven months but some dogs can cycle as early as every 4 months, some once a year. Lengths of heat cycles and intervals between cycles are different for each dog, but most dogs hit somewhere close to the averages.
The first part of heat you will notice bleeding from the vulva, swelling of the vulva, possible increase in urination and the most noticeable, male dogs hanging around the house. During this period (proestrus), females will not allow the males to breed with them although the males will be very persistent.
The second part or estrus is the time in which the female will allow the male to breed her and this can last anywhere from 4-21 days. A female, most of the time, will allow most any male to breed during this time. As the female starts to go out of heat or enter diestrus, she will be less willing to breed. Again this stage can last 4-14 days but averages approximately 7 days.
The next cycle usually begins about 7 months from the start of the last heat cycle, not the end of that cycle but again this varies from dog to dog. The interval stays the same even if she becomes pregnant.
If you do not intend to breed her, you really should consider spaying her. There are many health benefits associated with spaying such as decreased chance of mammary tumors and you will not have to worry about pyometra, metritis or unwanted litters (overpopulation is a big problem - just take a trip to a humane shelter). If you elect to spay her, this can be done during heat but it may be best to wait until the heat cycle is over to decrease the chance of complications.