The absolute furthest that anyone can sell it as reaching is probably in the 2070s (fred franz died in '92, so someone that was 20 and newly anointed at that time could live to be 100 putting you in 2072) but I suspect it's going to be discarded in favor of new doctrine well before that. The current GB won't last that long, so when some new folks come in they'll implement their own ideas and will probably scrap the overlapping generation, and I'm guessing 1914 with it.
If I were still in the cult, I'd probably be thinking that if armageddon doesn't come by 2034 it would prove them to be wrong yet again.