A dog will enter her estrus cycle from one to three times per year and it lasts anywhere from 2 to 3 weeks. How often a female dog menstruates and for how long is dependent upon breed. Some breeds will have an estrus cycle only once a year while others may have a cycle three times a year.
Dogs do not have seasonal estrus cycles. Once a female puppy begins menstruating, it can take almost 2 years before there is any type of pattern with its cycle. A female dog is most fertile at the point in its cycle when the discharge is watery, usually near the end of the first week.