In Noah's day, 120 years was perhaps one quarter, maybe more, of a lifetime of someone living then - so many living would witness the whole ''process'', it WOULD be within their generation. What logic is there in applying 120 years to today? That's almost twice a lifetime.
Bingo! I've long thought the same thing. According to Genesis, Noah was 600 years old when he went into the Ark. Assuming this story is accurate, that means he spent less that 1/4 of his life preaching. So if you apply this logic to us today, if the average age for a person is 80 years, then they should not have had to preach any more than 20-25 years before The End came. But of course as we all know, this hasn't happened. For 135 years, the WT has been saying that The End is "soon", yet how many generations have come and gone and never seen it?