How long has it been since they pipped? Ducks are very, very slow hatchers sometimes so just be sure before you help that everything is absorbed. You could start to help them out if you are concermed they are stuck but stop at the first sign of blood. If you get them out and it turns out the yolk hasn't been fully absorbed, you'll need to do something like this:
Put the baby back in its shell and wrap the shell with moist paper towels. Cut the corner out of a ziploc bag and put the duckling in its shell in the bag and pull its head through the corner you cut off. Then seal the bag. This keeps everything very moist and makes it so the duckling can't pull away from the yolk and out of the shell before it is absorbed. There's also another method like this that you can do using a cup but I've never tried that. Hopefully this won't be necessary but it's good to know just in case.
About 24 hours I think. I am able to see in the egg and can see that the membrane is tight around it but could carefully peel it back with tweezers if i needed. What do you think?