It's hard telling. If they are fertilized you should start seeing changes in the eggs' color in a week or so, definitely within 2. Three weeks ought to allow you to see features of the snails inside the eggs. Worst case scenario is that after a month you've got a bunch of rotten eggs on a sponge.
They appear fertilized. You need to keep them moist. I would put them on a plant float or a piece of the foam and let them float on top of a covered tank. The condesation will keep it moist. It usually take 2 to 3 weeks for the snails to hatch.