Please do not breed her. Take her to the vet ASAP and get her spayed. There are no good reasons to breed a DSH, period.
As for your question, she will call as long as she is in heat, and possibly in between heats. SHe will also become a nightmare to deal with and be constantly trying to get out. AT 8 months she is quite capable at getting pregnant, but still much to young to have a healthy litter (she is still a baby herself!)
I would think if you were going to responsibly breed a cat, you would have researched a bit more and would know the answers to these questions already. :?