The Copyright Act of 1790, just a few years after the constitution was signed, enabled a 28 year copyright grant (14 years with a 14 year extension.) So unless you think the copyright laws haven't been legal since their inception, you probably want to bump your number up at least five years.

Edit: Also, if you're interested in this argument, and IIRC the most recent Supreme Court opinion on it is Eldred v Ashcroft (2003).