Here is the primer on this.
The Trinity study established (with a few other repeat studies showing similar findings thereafter) that you can spend 4% of your "nest egg" (what you have saved for retirement) and rest reliably assured it will last for 30 years (96% chance).
From this, a reciprocal rule came about called the "25 X" rule meaning at whatever point you have 25 x your annual spending, you are financially independent. So, if you want to spend $100,000 per year, then $2.5 million is your financial independence number. If you wanted to spend $200,000 per year you would need $5 million. All of this is predicated on annual spending. So, if you can find contentment at a lower end of the spending spectrum and have a high income, financial independence will come quite quickly.
Remember, at this point there would also be no debt. No mortgage, college for kids, cars, or student loans. In that situation how much do you need to be happy? Studies suggest that number is somewhere between
$75,000 and $105,000 depending on the area of the country you live in.
That said, if you want to be safe (and retire really early) you might multiple your annual spending by 30 and know for sure that it will last so long as you don't undergo the worst sequence of return risk in know history (i.e. market returns drop really low for your first decade in retirement).
Others prefer to build passive income and when their passive income (say from real estate) matches their annual spending, that person would also be financially independent.
I take a
hybrid view of financial independence where I intend to get to my "25 X number" in addition to having some passive income streams that should float me if there is any trouble.
It is not unreasonable what your ortho friend says. I am anesthesiologist and plan on being financially independent in my mid 40s and finished training at age 32.