A dog’s menstrual cycle and duration vary depending on the dog’s breed, age and individual differences. Typically, a dog's menstrual period lasts between 2 and 4 weeks, but some dogs may have periods that are slightly longer. However, if your dog's menstruation has lasted for 40 days and there is still bleeding, this may be beyond the normal range, and it is recommended to take your dog to the veterinarian for a check-up as soon as possible.

There are many reasons that may lead to prolonged menstruation in dogs. First of all, the length of menstrual periods varies among different breeds of dogs, and some breeds of dogs may be born with longer menstrual periods. Secondly, age is also a factor. Young dogs usually have shorter menstrual periods, while older dogs may have longer periods. Finally, some physiological problems, such as endometritis, uterine fibroids, or other uterine diseases, may also cause menstrual periods to last longer.