Ducks are only good house pets as long as you can keep them comfortable in a large box or cage. As the ducklings mature to about 6-8 weeks of age, their care demands will change such that they need access to water, will become a bit messier, and will require more space.
Generally, we consider the 6-8 week old mark to be “pond ready”, when ducks can be released as primarily yard or pond ducks. Granted, they’ll still need some type of duck housing, but their domain should move to a more natural environment at this age.
Now there are certainly exceptions to these guidelines as there are for nearly any bit of pet waterfowl care knowledge. We receive many testimonials of pet duck owners who keep their friends primarily indoors, but most breeders and experts will agree that they need to be more of a yard pet.