Maltese need to pee 3 to 5 times a day. At minimum they need to pee once every 8 hours, but letting them out more frequently than that is better.
Maltese puppies have very small bladders so they need to pee once every hour or so. Maltese puppies will also need to pee after they wake up, eat, drink, and play.
A doggy door is a good way to ensure that the Maltese puppies or even adult Maltese are allowed to pee as often as they need to.