Rarely. I have snails spawn and lay eggs in my system pretty constantly, but almost never have small snails coming out of the process. It all usually turns into free fish food.
Did you add anything new to the tank recently? I once added a piece of live rock that I bought at Petco, which I bought because it had some snails on it and I was straight outsmarting the poor teenager who was there, and when I added it TONS of stomatella snails poured out of it. Did a great job of keeping my tank clean for awhile until they ran out of food.