Hi Miss Ducky!
The Harnesses hold half of a disposable baby diaper (We just cut them in half) and should be changed at least every 2-4 hours, depending on if/when the duck was eating. If the duck was eating or drinking while wearing the diaper harness (Or if it ate/drank right before) you will have to change the insert sooner/more often. If the duck is not eating or drinking, I usually change them every four hours or as needed. They can poop a couple times into one insert before needing a new insert.
I've heard of many people with indoor ducks (who don't give their ducks access to food and water at night) diapering them all night long (about 8 hours). My ducks have a pen, so I've never had to do this personally. There is enough room to lay and egg in a diaper (that's actually how I found out Victor was actually a girl, lol). Their tails can either be left out or tucked into the harness, but if you are anticipating that an egg will be laid soon, you can leave the tail feathers out to give it a bit more room.
Hopefully that answers your questions! If you have more, feel free to ask here or PM me!