It actually lasts about three weeks, if I remember correctly. Usually the second week is when they start bleeding. But it can vary.

Gray Dawn Treader answered on 11/30/12. Helpful? / 0

A dog's heat cycle lasts approximately 21 days. The first part, the heavy bleeding portion, is usually from day one to day 8 or 9.
Next, the bleeding slows down or even stops and the vulva becomes extremely swollen and soft. IF there is still bleeding, it is usually much paler and pinkish. This part lasts normally from about day 10 to day 16 or so. THIS is when the dog can be bred and is the most critical period to watch!!!
Finally, there is a return to a dark red/brownish discharge and this portion lasts approximately four to five days. The female WILL still be attractive to males and needs to be carefully watched.
The most important thing to be aware of is that these numbers are only averages... I have had dogs get pregnant as early as day four and as late as day 22. And, each heat is different in the same female so what happened last time doesn't necessarily mean it will be the same next time.
Therefore, you need to keep your dog confined for a minimum of 21- 24 days!

Toto, CD, RN, CGC answered on 11/30/12. Helpful? / 1