University of Cambridge economist Rufus Pollock has applied the science of economics to the question of the optimal copyright term. From the paper abstract:

The optimal level for copyright has been a matter for extensive debate over the last decade... A parsimonious theoretical model is used to prove several novel propositions about the optimal level of protection. Specifically, we demonstrate that (a) optimal copyright falls as the costs of production go down (for example as a result of digitization) and that (b) the optimal level of copyright will, in general, fall over time. The second part of the paper focuses on the specific case of copyright term. Using a simple model we characterise optimal term as a function of a few key parameters. We estimate this function using a combination of new and existing data on recordings and books and find an optimal term of around fourteen years. This is substantially shorter than any current copyright term and implies that existing copyright terms are too long.

Here's the paper.

One reaction I had was, "Hm, fourteen years - perhaps not a coincidence that he ended up with the same result as the U.S. founders originally set? Could the parameters have been tweaked to yield that result?" The formulas have four empirical values as inputs: the ratio of deadweight loss to welfare under copyright (α), the rate of diminishing returns as the number of works increase (γ), the rate of "cultural decay" which diminishes the market value of a work over time (β), and the discount rate (δ).

With various combinations of inputs based on values estimaed by other researchers in pervious literature, Pollock derives copyright terms from 2 to 50 years; to justify today's copyright terms which average around 70-120 years (longer if human lifespan grows) would necessitate implausibly extreme values for these variables.