One reason the duration of protection from childhood polio vaccine is unknown is that there has been no case of polio acquired in the US in over 40 years. I assume similar statistics apply in other countries that have very high rates of full vaccination. With the virus apparently circulating in NY--despite no new clinical cases other than the case of the unvaccinated person who acquired the infection overseas--we may get to find out if protection wanes.
Of course if a vaccinated person gets polio it may be because the person is severely immunocompromised, not because the vaccination was too long before.