Figure out how much money per year you'd need to live comfortably.
Comfortably meaning: enough money where your basic needs aren't a concern. So you don't worry about the rent or mortgage. You can walk into a Whole Foods and not check your bank account while you're on the line to pay.
Depending on where you live in the country, that might be like $150,000 a year. Maybe more, especially if you have a family.
Multiply that by how many more years you think you might live. Average lifespan in America is around 78.
Let's say you live in Los Angeles or Miami and you're 35 years old. Maybe $200,000 a year to live there without worry. You're looking at about 9 million dollars. And I think most people would squander a good portion of that so maybe add 30% to whatever your number is.