A heat cycle lasts for 3 weeks with any breed. It's one week going in, one week in and one week coming out. Keep your puppy separated from all males that are not fixed. You don't want her getting pregnant, because she's way too young. The best thing to do is to have her Spayed. It will keep her healthier in the long run and will eliminate the possibility to get certain types of Cancer.
It last for 21 days app 3 weeks and should not be breed this time she is very young and not mature yet.