and if they don’t? and how long could one just stay here without getting caught?
Getting caught is a variable time frame, and irrelevant. Overstaying a visa by one day is a crime everywhere, and becomes a crime the very day that person stays beyond the visa’s term. And the person will be deported when caught... or that is what is supposed to happen under law.
Not sure where you are going with this. Are you unaware of said laws?