How long do dogs periods last?

With dogs its called "being in heat" rather than "being on a period" but that aside, a normal female will be in heat for around 21 days, but is only noticeable for about one to two weeks.

The length of a female dog's heat varies, but is generally around three weeks. She is not receptive to mating during the first week or so, which is usually when the most spotting of blood occurs....

3 weeks

Dog's menstrual cycle lasts 3 weeks in average. But some dog's cycle can last only in 2 weeks, while some last up to 4 weeks.

The average is three weeks or 21-days. In some dogs, it lasts only two weeks while others go four weeks.