The heat cycle last for three weeks or 21 days.
The first stage called proestrus last for 4 to 9 days. During this stage you should see a bloody discharge.
The second stage called estrus last from 4 to 13 days. At this time the discharge will turn yellowish in color. This is the prime time where your dog can get pregnant. During this stage she will let a male mount her. She will do what is called "flagging" which is moving her tail up and to one side when approached by the male.
I do not recommend breeding her at this young age. She is still a pup could have problems delivering and taking care of her pups. If you plan on breeding her I would wait until her third heat or at least 1 1/2 years old whichever comes first.
To be safe, I would keep her away from male dogs for the full three weeks.
I hope this answers your question. Good luck.
