If you needed say $50k a year and Bitcoin didn't have volatility but grew at a stable 50% a year, than $100k would be all that you needed to be able to live on it forever. But that's really tight and not good at all.

Since there is significant volatility I would say if you can manage to acquire $250k of Bitcoin, $50k would be 20% of your stack, then you probably could manage to make it last forever considering you only sell what you need in a given month.

This is assuming of course that you didn't acquire that at the peak of a bull market which then crashes 85%... We're nowhere near the top now though, so really anyone with 2.5 Bitcoin or more is set and just doesn't know it yet because they're still thinking in FIAT retirement terms of 5% a year withdrawal when this beast grows at 10x that.