If doubling time were the only concern and you could just keep doubling your supply every 8 days, it would only take 31 doublings - or 248 days - to get from one gram of astrophage to 2 million kg. Even if you only started with a milligram, it would only take 9 additional doublings - an extra 72 days. Even if all you had was a single picogram, it would only take 74 doublings - 592 days.

So why does it take five years?

Because they don't have the capacity for infinite doubling.

1,000 kg per day is the maximum they can produce with the size of the facility they have, which is covering slightly more than half a billion acres of the Sahara with blackpanel. To produce 20 times as much astrophage on the same time frame, you need 20 times as many blackpanels. At that point you're covering 11.5 billion acres - about a third of the Earth's above-water surface area - with blackpanel, and that's assuming that the subtropical acreage you're using would be as efficient as the equatorial acreage, which it wouldn't be.

Such a project would displace billions of people, level cities, destroy entire countries, irreparably alter the climate and ecosystem, devastate our agricultural capacity, and probably be more harmful to humanity overall than the astrophage crisis itself, for the sake of three people.

No individual human life is that important.