Basically, the Copyright Term Extension Act wants to add another 20 years to the copyright. (before this, it's author's life time plus 50 years, or plus 75 years if it is corporate authorship.) This is proposed by the US government and passed into law. (supported by big corporations, e.g. Disney, Movie Studios etc.)
The “Eldred v. Ashcroft” is a case where people who are opposed the CTEA but failed. (they include, for example: universities, public domain publishers, internet users, Free Software Foundation…).
Regarding what is the proper period, we should reduce it to perhaps the simply 50 years from publication. The essence of that is to focus on individuals. Copyright law is there for the benefit of society. Copyright creates the incentive to create. (with the premise that human animals, like all animals, are motivated by self-interest) Copyright protection has a limited duration because once the author is dead and can no longer benefit from copyright, releasing the right would benefit the society at large.
Current duration of author's life plus some 70 years after the author's death, is ridiculous. This means, the benefit focus ceases to be primarily for the author himself, but rather is focused on entities that has little to do with the work's creator. (e.g. corporations, remote relatives (“estates”).) Also, a law of long-lasting copyright will have a effect of delaying the onset of commercial profitability of creative work. (worse scenario being, creative work will become commercially appreciated only after the author's death. See: Support Living Artists.)