When you get the visa stamped it will say how long its for. Normally it would be given until the expiry of the passport. So if its 10 years left on passport they might get it for ten years. Meaning they can come any time, 3 years from now, 6 years from now 9 years from now etc.
If for whatever reason the visa officer gives less than passport expiry like maybe 1 year. I would recommend using it, following all the rules and returning on that visa within the allowed time. This will show that they follow visa rules and give them travel history so next time they apply they might get maximum visa.