I don't think I'm smart enough. I'd just take the number 8 and double it. Then see how many times it takes by the same doubling to get to 6 billion. I think it's between 29 and 30. If that were generations of 40 years, then it would take c. 1200 years to get 6 billion. Maybe there are others who can help here.
I wasn't the one who asserted the impossibility. But I did ask for the math. So far I don't think anyone has supplied sufficient evidence to substantiate the impossibility of it all.
Unfortunately, it's way more complicated than that. I also think that you have to start with 3 instead of 8, since that is the number of fertile women on the boat. The fact that people die also has to be taken into account; infant mortality rates, longevity, disease, war, etc.
Too bad AmishDude isn't still on, as I think he might actually enjoy doing something like this. It is certainly way over my head, which is why I went looking around on the internet to see if anybody had already crunched the numbers.
(AmishDude: we were discussing how long it would take to repopulate the earth after the alleged flood, FYI).