They can sometimes take more than 24 hours from the first little crack in the egg shell until they are unzipped and hatched- Some are quicker than than that- but generally I find 24 hours to be a good estimate.
I wait at least 48 hours. That's about how long mine take to hatch from when they pip. It's so reliable in my flock that I start contacting the folks on my duckling reservation list as soon as they start to pip to tell them when they can come pick up their babies. Also, it's sometimes up to 48 hours between when the first one pips and the last one.
If they haven't done anything after 48 hours, then you can start to worry. But it would be highly unusual for that many eggs to pip and none of them to hatch, unless something has happened environmentally (like an extreme, prolonged temp spike). So give them a little time, then think about pulling one out to see what's what.
Mine have varied from a few hours to 48 + hours -- tho' more have successfully hatched when out earlier than late so I do get nervous. It is sooooo hard to be patient, but try to remember that they've got a little yolk sack to help them keep their energy up for a couple of days (and they do rest for long periods sometimes). I've not hatched in an incubator though, always under broody ducks and I can't resist but peek a couple times a day when the time gets close.