You're counting the years per bit incorrectly, the comment you reply to gets it right - the new generation is needed when the current space runs out, not when t

You're counting the years per bit incorrectly, the comment you reply to gets it right - the new generation is needed when the current space runs out, not when the next limit is needed in its entirety. So the ~18 years you mention are the time from needing 33 bit addresses to needing 49 bit addresses. (And 7 years is the time from needing the 9th bit of address space to needing the 17th).