33 * Cost of Living / (Salary - Cost of Living)
Assuming for simplicity 3% real interest on investments once you stop working (which is close enough if you're trying to get out of the rat race quickly), and potentially infinite lifespan.
So if your cost of living is $10k and you earn $100k you get:
330,000 / 90,000 -> less than four years.
Great theory.
I don't want to live on oatmeal and tuna for the rest of my life. (I actually don't think we should be eating tuna at all and have stopped eating it although I love it.)
Now aside from obvious (e.g. kids, as you state) there's also:
* At some point you'll get a $500 optical bill or a $5000 dental bill (or a $50,000 medical bill if you live in the US) -- and let's not even get into chronic conditions.
* You may want to travel.
* At some point there will probably be a major economic shock and you may be utterly screwed. (But hey, so will a lot of people, and you'll be used to it.)
Finally -- you may enjoy working. I do.