~640k won't even buy you a house in a lot of the US. 500 million yen or 3 million usd would be the minimum for someone who doesn't want to work
Somebody here did do the math huh.
It also depends on the lifestyle and location; I've had to explain to some naive (idiots) people about the simple math and banking/interest rates. Working at 40-60k a year isn't gonna get them to retirement and even 100k a year (under the current US administration) isn't going to either. We are seeing the highest inflation rate since the Regan era and possibly a permanent divide that will never see the "middle" class recover.
3 million at 4%/high interest savings account is about 120k (taxable) passive income per year. those savings rate at subjected to the bank's whim and could be changed at anytime. Bonds are also possible, but those are typically only 1 or 2% and will not keep pace with inflation. Passively managed funds (ETFs, Mutual funds, Hedge funds) are also an option and usually the highest returns, but ultimately depends on the fund manager's competency and honesty. The return rate of a competently managed fund is 10-14% annually, giving it the possibility of doubling a fund's wealth over 10-20 years. But if the country the fund is based in goes kaput. Well, nevermind the dollar, one would be looking at the collapse of modern civilization.
Welcome to late-stage capitalism.