1) Suppose that "intellectual property" (IP) is taxed, like real estate. Say at a T = 1% rate, to keep it simple. This pays for the property protection service the state provides.
2) But the holder does not actually pay tax. He just accumulates back payments to the state, at an interest rate R per year.
3) When the state is owed 100% of the value, the IP reverts to the public domain.
This has the advantage that the property value does not matter - it can be 1, 100, 100k; nothing to haggle there. It is just assumed to be constant for the period considered.
Run your own calculations. Mine were that with a T=1% tax rate, and a R=5% interest rate on unpaid tax, you get 37 years. (26 years for R=10%, 47 years for R=3%.)
P.S.
I got curious, and just checked with a lower tax rate T=0.5%. R seems to dominate T. Here is a little table:
R=3% R=5% R=10%
T=0.5% 66 50 32
T= 1% 47 37 26