That's interesting ! My very first clutches, from my first snails about 3 years ago, hatched very fast.. about two weeks. Then I had some that took a month. Reading online didn't tell me much, some sites said a month, some said less. In the summer months, I was getting new clutches almost every couple of days, sometimes three in one night, but they're certainly not laying that fast now. Even though their lighting has not changed, they are not far from a window so maybe the shorter days are affecting their reproduction.
The clutches I've had this year were hatching anywhere between 2-4 weeks, and lately, closer to the 2 week mark. I watch them to see if there is any sign of an egg rupturing or a bit of the egg case in the water, and if I see that, I'll remove the clutch from the glass and gently break it up in the water to let the rest of the little ones out.
Some of them have hatched entirely on their own, and all I find is the remains of the casing, but some seem to get, sort of stuck, I guess. I had a couple that never hatched and when I opened them to check on them, they had baby snails inside, dried out and dead. Since the glass top is always closed except for about a quarter inch ventilation gap, they aren't drying out where they are laid, so I'm not sure why that happened to them. So far, most clutches have produced baby snails, though I've yet to see one produce really large numbers of babies. Most have produced a couple of dozen or so, with one or two producing maybe 3 dozen or so.
I'd like to see more baby snails, because I can sell them easily. So I'll keep chugging along.
