Since my comment triggered a real discussion, I wanted to share some of my thoughts on the subject. For what it's worth, my own views are probably on the fringe side.
To start with, when I say "financially fully independent", I'm not talking about a fixed amount. Instead I'm eyeing a figure that allows you to live comfortably (but not lavishly) with a family, in the place of your choosing, and not have to ever touch the principal. Even in the face of a devastating decade-long recession.
Let's work backwards from that. In case of a massive global downturn, looking down the barrel of a universal lost decade, it is still possible to invest your money so that you can get a reliable 1.8% above inflation from dividends alone. That's pre-tax. Assume capital gains are taxed at 35%, and we're talking about ~1.2% annual real return on investment.
If you expect your family's annual, comfortable living expenses (with assorted housing maintenance costs factored in) to be about 110k net, then you'll need 110/0.012 = 9.16M to invest. Let's round that up to 9.2M.
In case you want to retire to a relatively lavish lifestyle, so you can get a large estate and enough liquid cash to pay for helping staff even under the most drastic but still realistic scenarios, we're talking about post-tax net income of >200k a year, and a principal of at least 17M.
Feel free to plug your own numbers in.