"Experts" in the US often say about 20 years at a $40,000/yr withdrawal rate. Withdraw more, or enter a recession, or require more time, and a million dollars isn't enough.
Actually, just living off say, 3% interest, and withdrawing $2000/month, you'd get a luxurious lifestyle here. It'll cover health insurance, bills, rent, a mid range car, eating at restaurants every day, plus a little extra to buy some nice toys.
(not investing advice, see https://www.reddit.com/r/ExpatFIRE/ for more info or other leanFIRE locations)
40 years old. A million pound would last me the rest of my life.
Assuming I earn ~£50k a year, lets say £35k a year after tax and deductions, for the next 20 years, that comes out at about £700,000 after tax income. I live comfortably right now off that.
That still leaves me £300k.
My outgoings are pretty small, and will be drastically smaller in 5 years or so when the mortgage is paid off.
If you give me a tax free million today, I would quit my job tomorrow, as that's more than I'll earn for the rest of my life.
That's not counting money I already have in savings too.