Doesn't really matter to me. The Predators films have no overarching story, and each of the three can just as easily be looked at as their own movie as they can as part of a series. If the new film and any future movies stay consistent in that they are self contained stories, it could theoretically go on forever.
As for how long it should go on before it goes stale, some would certainly argue that it's already reached that point. I'm interested in seeing Shane Black's take, however, and would definitely be open to seeing the franchise change hands a few more times to see what some other directors can do with it afterward.
