Dogs will continue to go into heat while their bodies are able to produce babies, or they run out of eggs. Every dog is different. Usually, if a dog is bred again & again, the litters get smaller and smaller as less eggs are available.
The best thing is to spay her. You'll be avoiding the mess of her heat, the annoyance of interested males, the difficulty, hard work & cost of caring for mom during pregnancy, unexpected c-sections (over $1000) and raising the puppies and vaccination them. Then the problems dealing with new owners who want to return the puppies because the didn't realize they weren't already potty trained or that they chew and possible harassment from animal Welfare Societies when someone complains about you being a backyard breeder. Sometimes you can be fined thousands of dollars for violations.
Even with no problems, you'll never make a profit unless you are breeding show quality puppies and even those breeders breed more for the love of the breed than for money.