[jstanley]

These are 20 people who live infinitely long, never age, and can work 12 hour productive days.

As long as there is no single step that requires more than 20 simultaneous workers, I don't see why it's not solvable with these 20 people.

[xjwm]

This also assumes that as each step is solved, it doesn't require a continued human presence. I.e. if you develop electricity, the electrical plant needs to be automated and run itself without human (or computer) interaction. The laws of entropy are going to come into play eventually. You might not be able to keep everything working for long enough to make meaningful progress.

[hawski]

And as long as this single step does not take more than 12 hours.

[saagarjha]

It will just take a long time, though…

[jstanley]

Naturally, but the original question was how long.