Low ones-of-millions - say $2M - is still enough to last 20 years at $100K/year (not counting taxes, but also not counting interest). Most decent entrepreneurs ought to be able to come up with another viable startup within 20 years, so for most practical purposes, it basically is fuck-you money. You won't be able to never work again, but you have enough of a financial cushion that you never have to work for another person again.