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

[Storm_Of_Snow]

If the process does not involve a supernova but simply rapidly squishing the Sun until it becomes a black hole then it'd be maybe a year before the last person dies of cold. This is because all that would be different is that we'd lose all of our precious sunlight, the Earth would cool down, eventually all the way down to about 2-3 Kelvin. We'd be dead long before that though, most would die within a month, a few survivors in underground bunkers could last as long as they have fuel, food and water.

Grow lamps and space heaters exist

The moon would still be orbiting the earth, so geothermal and tidal are still possibilities for generating power, while greenhouse gases would actually be beneficial to keep heat in. Throw in fission and eventually fusion, plus sealed environments, and civilisation could potentially survive for a long time.

Jupiter might be a longer term source of energy - end goal would have to be migrating to at least Alpha Centuri, so you'd need some colonies to start that process.

That's not to say there wouldn't be a lot of casualties from the initial event, and there'd be conflict as nations try and grab what they can to secure what their leaders see as the right way for civilisation to continue (their way and no other), which could well turn into a nuclear exchange.