I'm going to copy and paste from another post I replied to with a similar situation (except a chicken egg instead of duck egg but it applies to both)....
"Does the hole where he pipped seem to be turning tan or brown? There are two membranes. The outter membrane and the inner membrane. The outter membrane looks like paper and is right under the shell and usually what you see when the shell is cracked. Inside of that is the inner membrane. This is connected to the chick and holds the veins/blood that the chick is absorbing while its pipped. If you do need to help the chick out, you can't until all the veins have been absorbed by the chick. If you rip a vein in that inner membrane the chick can bleed out. Also, the chick is absorbing yolk too, which most likely won't be fully absorbed until the veins are. You can moisten that inner membrane and check for veins. This will let you know if the chick is ready or not."
If it reaches 48 hours or you think the duckling is In distress you can take tweezers and gently pull back some of the outter membrane from the pip. You can locate the inner membrane and moisten a qtip with warm water and apply it to the inner membrane (don't get any water in the ducklings nostrils). When the inner membrane is moistened you can see the veins more easily. You don't want to make a hole to big because the more you expose the inner membrane the faster it will dry out. If you need to investigate and check that inner membrane you can put some kind of lubricant (bacitracin, coconut oil, Vaseline) on the inner membrane. This will keep it pliable and soft, otherwise it dries out and gets hard and sticky. Just keep monitoring the duckling. If the duckling is yawning that means it's still absorbing the yolk. I've hatched ducks, it's a long process. Some of mine took 4 days. Ducks are twice as long as chickens. Can you see the bill or is it just a crack in the shell for now?