Well, are you talking about how long from mating till eggs are laid? Or are you talking from time the eggs are laid until they hatch? Also, what kind of snails are you talking about?
They might not even lay any eggs. I'm not sure exactly how long until they do, if they do though. But since they will be above the waterline, they will be easy to see. Depending on the temp of your tank, after they are laid, you have 1-2 weeks until they hatch.