If the duck's bottom is a bit messy when it comes time to changing (if they are not going into the tub) we recommend wiping it, with a natural/sensitive wipe just like you would with a baby. Normally they don't get poop all over their bottoms, but it happens sometimes. They can poop a couple times before needing to be changed.
Usually if we plan on keeping them diapered for awhile, (like on a car ride to the vet, or long bouts of couch snuggling) we don't let them have access to food/water other than peas as treats. This helps cut down on poops per hour, thus changing less often. I tell people to use the rule we use in caring for babies and toddlers in preschool.... If it sags or smells bad, change it. If it's been two hours or more, change it regardless as to how much poop may or may not be in it.
The very longest we ever go is maybe 4-5 hours without changing if they are not eating, since they don't go that often. But whenever they go for long bouts in a diaper harness, they immediately go into the bath tub when done.
To get back on track with the OP's original question.... When I first got victor, I was so sure he was a boy. While driving the 14 hours it takes to get from Chicago to Alabama (I lived in AL briefly) when the ducks were about 5 months old, I'd set up the back so they couldn't fall to the floor. I had a baby crib bottom folded out over the back and elevated them so they could easily see out the window. While stopping in Tennessee for gas, I noticed Victor was sliding around on his belly in the back seat. I picked him up to see if his harness was uncomfortable or if he needed a change, and low and behold, all I found was an egg. The very first one I ever got from any of my ducks, and it came from Victor. lol...
Basically we found that if you diaper your duck and you might expect and egg to come while being diapered, jst don't tuck in their tail feathers (let them hang out the back) so there is plenty of room for poop and/or an egg.