50 to 100 snail-ettes in that egg sac. They always lay at night, generally about twice a week, and always at the top, at or above the water line. Your snail can continue laying eggs for six months after contact with another, so just because the mate is gone doesn't mean you don't have to keep checking. Do you have a heater in there? One clue is the snail hugging the heater in the daytime. It will most likely lay eggs that night. They don't always hug the heater, but it's one way to know.
In the morning, check around the top of the tank, even up around the top of the filter. fresh eggs are totally soft and mushy. Messy to clean off with a paper towel or whatever. One day old, there's a thin crust on the outside. This is a good time to flip it free with your fingernail. 2-3 days, the crust is harder. It may taker a small knife, edge of credit card, etc to remove it. The longer you wait, the harder it is to scrape off.
About three weeks, you'll see the outside looking transparent with little dark dots inside. Those are your snail-ettes formed and about to hatch.