After 2-4 weeks (depending on the species and the temperature, 14 days at 25°C in the case of Pomacea canaliculata) the first little snails should appear. If they don't after 3 weeks, you can try to help the young snails. It might seem rude, but it works: just break the clutch in 2 - 3 parts and those in the water. You can even wash the snail out of the clutch by moving the broken clutch parts trough the water. Note that this isn't necessairly when the moisture of the air is sufficient.
If you don't see little snails after breaking a 4 weeks old clutch, it can be that the eggs aren't fertilised or that the air moisture was too low. In the latter case the yolk is very thick or the eggs are completely dried out and filled with air. You might want to wait until the snails appear on the natural way (it can take several days before they all are hatched), but keep in mind that if the clutch isn't kept moist enough, the young snails won't be able to make their way out and subsequently die.
The apple snail's life expectancy mainly depends on the temperature of their environment and the general life conditions. At lower temperatures, the apple snail can get over 3 years old and records of apple snails of ten years old have been reported. Lanistes nyassanus has an estimated life span of 5 to 10 years in Lake Malawi, Africa.
At a temperature of 25°C, they will only live 12-16 months. At higher temperatures, the snails are more active because their metabolism increases with the temperature, but this also speeds up their life cycle and thus shortens their life expectancy.
So you can elongate your snail's life by lowering the temperature. A general rule: keep the temperature between 18 and 28°C (65-82°F).
It is suspected that at least some apple snail species need an aestivation period in the mud to avoid burning-out
