I went through this recently, and determined that I couldn't retire early. Started saving too late, have too much going on now to put money into risky investments. Toss in whatever it will cost to pay healthcare insurance prior to 62 and I'm not sure I even want to retire early. I think I'll be content to just get everything paid off in 15 or so years, and then reach a point where, if I had to, I could take a severe paycut and just coast to retirement at 65-67 or so.
Inflation is the elephant in the room that no one wants to talk about. What will it cost 30 years from now? At least 2x if you ask me, and that's only 30 years from now, when I'm just getting retired! Many don't believe SS will still be around; I think it will, and it's supposed to be inflation adjusted: but will it really be?
I'm not sure what my vehicle cost would be in retirement; I commute about 21k/year but our vehicles get 50k+/year anyhow. My housing costs run 20% of my income--but my vehicles are running 21% at the moment. Two loans, ouch, I hope to not do that again (but probably will); even still, it's all the costs that add up (insurance, repairs, you name it). Getting rid of the mortgage will be of no help, not when it's less than 10% of my income. Which is why I wouldn't want to retire on less than 75% of my salary.
I figure I can't retire on less than $2M, not once I take into account inflation over the next 30 years -- which is basically today's $1M. And more would be better, of course. 4% of that, plus SS, should about do the trick, I think, in tomorrow's economy. Assuming no massive inflation.
I've heard that property is where to put one's money, but right now, having bought in 2005, I'm still too jaded to look into that option. Houses fall apart too, and I'm useless with my hands, and even worse with people, so buying rental properties and getting someone to manage them seems like a poor option for me also.