Per https://www.marketwatch.com/story/this-is-exactly-how-much-i..., retirees have a lifestyle of roughly $60k or so per year. At a more modest $50k, and assuming a portfolio of working assets from which to draw this $50k, that would be something like a portfolio of about $830k at 6%. Assuming a 6% yearly net gain on investments, then if you can somehow sock away about $33k per year, you might achieve FIRE status in 15 years.

So there you have it: The event horizon is based on how realistically you can soldier on and invest 66% of your target retirement income each year for 15 years. Not sure how realistic that is for most people, let alone this hypothetical 40-something in the doldrums.

I'm using 6% for sake of argument for no good reason, but it's the usual rate used in converting a pension into a lump-sum. It is possible to do much better - stock market has been about 8% to 11% on average. So maybe 6% works as a fudge-factor in this sort of planning versus life's many ups and downs.

Anyway, if one had 30 years towards this "$50k FIRE", then the yearly "at 6%" investing is $10k; at 20 years, $21k; 10 years, $56k.