It only works for people to whom the Wizard gives the power too. When Billy and Mary say the word, they are invoking the name of the Wizard, and they then receive the power. Capt. Marvel Junior, on the other hand, was given his powers by Captain Marvel himself, which is why HIS magic word is "Captain Marvel." The main problem these three have is that the magic word always works - they can NEVER say the word without the magical lightning striking.
Technically, there is no limit to how long any of them can remain in their super-forms. It is simply a mark of their sense of obligation and responsibility that they feel they should not weild the power unless there a real need for them to do so.
And yes, Black Adam had a similar gift. Lacking any sense of responsibility, he never chooses to change back into his "mortal" ID. In the original incarnation, this was because he was a 3,000 year old Egyptian warrior, so transforming to normal would have killed him.