ALL female dogs will have a heat every 6-9 months depending on the individual dogs cycle patterns. I would strongly urge you to spay her before she goes into heat ( usually by 4-6 months)
But if you are planning on breeding her even just once, please do your research first.
A female should never be bred before she is at least 2 years old. This translates into 4-5 heats! Other wise she could act like a kid who needs to take care of kids, and YOU can end up with a BIG CHORE of raising 8-15 puppies, who needs feeding every hour on the hour!
Here is another important reason! Large dogs are not fully grown until they are at least 2! So breeding her any sooner will put a tremendous strain on Her body, and cause her to be smaller than what she would've been. Not to mention bone density issues; hip and joint issues and internal reproductive issues, and DEATH. Breeding her too soon might very well mean C-Section too because she just wont be large enough to push out a 8-15, 1 pound babies. And if you love her, you will never risk her health and her life like this.