Heat cycles are generally 3 weeks long. First week coming in, second week is when fertility occurs, and third week they're going out. Not all Newfs have read the book, however! One of mine consistently was fertile around day 20. The only way to know when fertile is to run progesterone tests which are pricey and unnecessary if the bitch isn't going to be bred. My advice is to keep the bitch leashed when outside for at least 3 weeks. Unleashed, even in a fenced yard, can result in a breeding. Males can smell a bitch in heat from miles away and will jump fences to get to a fertile gal; bitches have been known to jump fences to be bred.
While this sounds like a hassle, remember that you are doing all you can to avoid the health issues an early spay can cause. She will thank you for it.