Here is what the Constitution says (pdf) about copyright and patent:
To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.
As always, the Founders recognized that times change, and that it is best to draw broad guidelines rather than precise definitions of stuff like how long monopolies on developments in science and useful arts should be in place. But they did clearly say "limited."
Current law doesn't reflect any notion of "limited" times.
The duration of copyright in these works is generally computed in the same way as for works created on or after January 1, 1978: the life-plus-70 or 95/120-year terms apply to them as well. The law provides that in no case would the term of copyright for works in this category expire before December 31, 2002, and for works published on or before December 31, 2002, the term of copyright will not expire before December 31, 2047.
So Mickey is good until 2048, at which time Disney will have gotten another renewal.
The entire point of having patents and copyright exist for a limited time is so that the work can eventually enter into the public domain, that the creators get a period of time where they exploit their ideas in a monopolistic environment, but, at some reasonable time after their invention or creation, the work becomes publicly available. The idea of copyright and patent is to protect innovation, for long enough that it encourages such innovation, but not so long that it remains out of the public sphere.
This idea has been lost. Walt Disney is long dead. He isn't around to take advantage of his ability to develop a new Mickey Mouse character. The corporate inheritors of his legacy of genius are stifling innovation, in violation of the intent of the Founders. But they are well-funded, and fund their legislators to keep Mickey, and every other work that took place after Mickey, out of the public domain.