I think "life" is sufficient. With that, someone can capitalize on their work for their entire career. Anything past death simply serves to allow offspring to ride the laurels of their parents.

As for corporations, I'd be open to a pay-as-you-go system like you propose, only I'd make the payment grow at some exponential percentage. Say the first year costs $1 and the fee grows by 25% a year. Adjusted for 3% inflation, the fees would be something like:

  Year  Fee/yr
  1     $1
  10    $5.71
  25    $104.17
  50    $13,169
  100   $210 million
  150   $3.36 trillion

This would put a natural limit on corporate copyright, due to their willingness to pay. With this system, Disney would pay $5,659,799 this year for the Mickey Mouse copyright (assuming it started at 10 cents in 1928, which is assuming 3% inflation backwards from now). By 2033 this would be a billion dollars a year... by 2064 a trillion. (Again, at 3% inflation, a trillion dollars in 2064 is like $200 billion today).