How long would the Earth last if the sun became a black hole?

[Ionathus]

I don't know whether we would have the time to construct new underground habitations before the surface becomes deadly. Ocean tides would be much harder to use when the top few hundred meters of the seas have frozen solid.

Yeah, this is my thought too. I think any proposal that "a small population could survive off geothermal", while technically true, isn't realistic unless there was a huge advance warning. The sun isn't just "a" heat source: it's THE heat source for our planet. If the sun went out suddenly, the surface would quickly become outer-space levels of dangerous. We're not talking about "oh we'll have to supplement with nuclear or geothermal" -- we're talking about everything that isn't meters and meters belowground needing to be its own self-contained life support.