Probably a few hundred years, so yeah, it wouldn't be worth it to the people doing it at all within their lifetime. That's why I was suspecting a far-sighted government or such with enough money to squirrel it away for future use.
The US sorta has a few things like that already--the oil reserve & the stockpile of helium, though I understand the latter is winding down still. Given the importance of those materials to science, I would think that there might be some scientifically-motivated project to protect our access to such things.
