The problem with sir82's point is Franz was 99 when he died and that was in 92 but that is one situation...surely they could not based an overlapping generation theory on one individual?
So if you say Generation 1 were around 18 when they became anointed, then Generation 2 would be around 18 when Generation 1 people were dying. So on average, Generation 2 people must have been born by 1948. That means that your average Generation 2 person is now about 65. And since we're using this average lifespan of 70 years, that puts the limit of when the end will come to sometime in the next 5 years.
The anomaly is that there were people like Freddie Franz. He lived to be 99. So when the end doesn’t come in the next 5 years, which of course it won't, the dubs will still be able to rationalize that Freddie died in 1992,and so Generation 2 must include some people who were 18 and anointed when He died, which pushes the outer limit of Generation 2's lifespan way back to 2044 (using the 70 year lifespan of people born in 1974.)
And... since none of the current GB will be still around at THAT outer limit, then they don't really need to care how to extend it any further.
But for right now, using the average lifespan of 70 and the average age of anointment as 18,that means the Overlapping Generation understanding gives the dubs roughly 5 years before they have to start reconsidering the whole thing.