Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are curious about your dog's heat cycle
and the typical length of a cycle.
It depends upon the dog but in general we see a bloody vaginal discharge lasting from 5 to 10 days, then a straw colored discharge with short period of fertility of 5 to 7 days, then the "going out of heat", although still attractive to males, period of 5 to 10 days.
So a heat cycle can be as short as a little over 2 weeks to almost a month.
The fertile period is USUALLY in the middle section. I wrote usually in capitals because every dog is an individual and I have known dogs that have gotten pregnant even when bred "too early" or "too late" in their cycle, at least as far as external signs.
If a female will stand to be bred then it is very likely that is her fertile time.
There are some very dominant
females that won't stand to be bred no matter where they are in their heat cycle, and sometimes the only way to get those pups pregnant is artificial insemination based upon vaginal cytology. This involves looking at a smear of cells swabbed from the vagina under the microscope to try to determine their fertile period by the changes in cellular morphology.
If you don't want unplanned pups then I recommend confining your girl, with no possible access to males, from the day that you notice a blood tinged vaginal discharge and vulvar swelling for at least 1 month.
Please let me know if you have any further questions.