Deathwatch by Steve Parker isn't what I would think of as 'older fluff,' mind. It was only published in 2013. Space Hulk 1st ed is, though, but 80 years actually matches precisely 100 years, since we are talking about the time from first implantations to fourth generation instead of from first implantations to purestrains.
If you have enough initial implantations, with exponential "infections," yes, you end up with a huge number of infected. And with another 20 years [making that 100], you have a tide of purestrains born.
But that doesn't require gestation of weeks. Each generation in Space Hulk 1st edition [published 1989, so yes, old fluff] takes about 20 years to mature and have kids, using DArquebus' citation above. So.. that's the same. The Fall of Darvon VI starts with a single genestealer implanting 10 human "brood brothers." A year later, they each have a kid [1st generation], and another year later they have a second. Eighteen years later, those likewise each have kids with other brood brothers [birthing the second generation], and eighteen years after that again [third generation], and only eighteen years later those third generations have the fourth generation [who look like normal humans, except bald and staring-eyed], who, in only eighteen years, lead to a mass of purestrains, following the math in the Space Hulk book.
Eighty years to fourth generation hybrids being born, and 100 years to purestrains. That's from Space Hulk, though, not from Parker's 2013 book, and GW has been known to ... retroactively change things [Necrons], and don't always care about math when they do so.
And assuming that Space Hulk math means they would overrun the planetary population of Necromunda forgets that non-genestealer humans are breeding at about the same rate, dying to violence about as often, and starting with a much higher starting population than 10. The planetary populations in WH40K, including in Necromunda, are Heavy-Metal-Gothically Obscene in their size. Incomprehensible.
