It's difficult to conceive how some entities even handle weapons of this kind as they are much more than foul play or a stroke under. All wars are the victory of the brute force, where destruction and tragedies fight against collective prosperity and development. Now, with thermonuclear weapons, the consequences would be even more catastrophic. The radioactivity remains for a long period, creating a “exclusion zone”, like it has happened before.
In Chernobyl, a “level 7” nuclear tragedy, the radiation covered zones of thousands of square kilometers. The residual radiation is still being studied after near thirty years. It includes groundwater, rives, lakes, and of course living organisms.
The numbers for safe levels are in the order of thousand years (surpassing 20.000) in the critical area according to the Ukrainian government. Public access is completely restricted and this is another point to remember. The same source cites increased points of contamination due to the radioactive decay of plutonium-241. It occurs something with similarity to this, so the calculations are based in radioactive decay for specific atoms.
More factories of this kind, shouldn't exist after knowing this experience. Then, more than 20.000 years is the answer to the initial question. Nuclear power inside the planet is not a development, but a nightmare that could end without waking up.