Good question. Over time, snails tend to die, and other inverts disappear. But also, over time, the need for a cuc gets reduced. As coraline algae covers the rock, it keeps pest algae from growing, reducing the need for snails and hermits.
Others may disagree, but after the first year, you only need a handful of snails and hermits to keep things maintained. Maybe a quarter of what you started with. I don't get to scientific about it, just go by observation. If you have a sanded, that does require either a full number of snails and conchs or whatever, unless you are diligent about keeping siphoned. Long story short, just go by observation.