In my opinion, someone hoping to retire early (before 60) needs to have enough cash on hand to last until any pension/SS kicks in AND without touching any 401K type funds. I retired at 50 and had investment accounts outside of 401K, plus cash on hand, plus a pension. I have a tidy amount in a 401K as well.....none of which will need to be touched until mandatory withdrawal must begin. That's the plan anyway. In terms of how much cash, figure your expenses per year (is it 60K, 40K, something else?) then multiply by the number of years you need that money to last....that's how much you need (and be generous because interest rates on "safe" investment vehicles are in the tank, while inflation is still a thing). In my case, my home(s) are fully paid for, I own rental properties which produce a steady stream of income, and I get a pension check. So far, we've had to touch very little of our savings (I'm talking the savings outside of the 401K) in the last decade. Because our homes are paid for, and DH works part time with full medical and dental coverage for the family, our expenses are relatively minimal. Property taxes, food and insurance (homeowners, etc) are our primary expenses which are "must" spends....we probably also spend a disproportionate amount on vacations and travel, but this is discretionary and easy to forego if we need to be "leaner".
To pay for college for our twins (the next and "last" big expense), we will sell our second home (which we purchased to be close to where they go to school). That will more than pay for college for both at a reasonably priced institution.
We've got 7 more years before we collect social security. That will become our "mad money" for travel, etc (if we ever, in fact, receive anything). DH will also get a pension check...not a big amount, but, heck it all helps.
If I was to retire by 40, I'd need a minimum of 25 or 26 years worth of money....either in cash in hand or some reliable stream of income. Very difficult to get any sort of "pension" by then because of years of service requirements, etc. Even with modest "needs" (let's say 40,000K per year in a relatively low cost area), that's 1,000,000 That's a lot. Plus, of course, that takes you only to 65. With retiring that early (40), your social security draw isn't going to be great. So, where's the money going to come from to live on after age 65? I'd say the answer is many multiple millions....at least for me to feel "comfortable" that I wasn't going to be destitute at the end.