(I'm not Petruchio, but:)
I think it's better to define "retirement" in this sort of context to mean (not taking some kind of vow never to work again, but) being free to choose whether to work and what on. If you manage, by some combination of accumulating wealth and reducing expenditures, to get into a position where you no longer need to work to live as you want to, and then find that you prefer working -- why, then, get a job! But then you have the freedom to turn down jobs that would be no fun, that you don't see as benefiting the world enough, that are too far from where you live, etc., while if you need to work to live you may not be able to get away with that. [EDITED to add: And if you get bored, or tired, and want a break, you can do so with less fear.]
A conservative estimate: If you assume zero investment growth before retirement, and consider you need 40 years' worth of money to retire (at which point, unless economic growth has completely stopped by then, you can take out 2.5% per year and probably see your capital growing handily most years), and if every year you save twice what you spend, then you can retire after 20 years. So he'd retire at 43 or thereabouts.
If Petruchio can increase his income faster (proportionally) than his spending, or if his investments grow at all over time, or if he is prepared to be a bit less conservative about how big a pile he needs before retiring, or if he expects to do a bit of paid work while "retired", his retirement can be correspondingly sooner. If he starts to yearn for a spendier lifestyle, or has children, or if there's a colossal economic crash, it'll move further out.
Like you said, that is a very conservative estimate, but entirely accurate. If we experience a horrible depression which gives me no investment returns for 20 years, I still will be able to retire at age 43.
The rule of thumb is that you need (Current_Expenses x 25). This is a 4% withdrawal rate, and should last you forever. If you want to go even safer (the Trinity Study suggests that 4% is plenty safe) go with 3% and multiple your expenses by 33.
Implication: You must save 25 dollars or so for each dollar you spend. So you go out and earn an extra 25 dollars, or figure out how to spend one dollar less.
The past few days have seen an increase of chatter concerning retirement and financial planning. One of us is even putting out a prospectus for a rational financial planning sequence. Some others have derided the concept of saving for retirement, as there is a probability of death before that time.
I am of the Extreme Early Retirement group. The idea is to save and invest 60-90% of your income, and you will have enough money to retire within a decade rather than four decades of the normal working career. This requires you to exercise your frugality muscle (such as cutting cable, biking to work, eating out less), but due to hedonistic adaptation, you will come out no less unhappy.
The sequences have already spoken on how spending money does not make us happier (after our basic needs are met). A Rational Financial plan should take this into account, even if a majority of people would not want to consider it.
I am just a beginner, so I linked the two big names in EEA, Mr. Money Mustache and Early Retirement Extreme. You can find their journeys towards financial independence here and here.
ERE is an austerity heavyweight, while MMM lives a pretty luxurious lifestyle, but still spends much less than his former coworkers. He just spends on what is important to him, such as travelling with his family and eating organic food, and not on anything frivolous, such as cable or eating out. He lives very far from a deprived lifestyle which the average person would shy away from. It takes a paradigm shift and some grit, but the people of LessWrong are not the type to reject munchkin ideas because it takes a little bit of mental effort.
If I were to make a compilation of posts for a Rational Financial Planning sequence, it will go as such…
How Little Money you need to Retire ?
Basic Retirement Math
Rationalist Spending
Maximizing Utilons per Dollar
Utilons Free Of Charge
Investing Rationally Basics
These are just the basics. Investment advice is scare, and the above does not talk about many fianacial aspects, such as insurance, children, career choice. The authors do speak about them on their blog’s, but I omitted them for brevity. Read and follow these posts however, and you will be better off than 90% of your peers, and well on the road to Extreme Early Retirement.
[Edit] This idea of cutting your expenses and maximizing your savings obviously do not apply only to early retirement. Other financial goals, such as saving for a house, building up capital for a business, or giving more money to charity all will be more quickly accomplished if you learn to cut excesses from your life. The driving idea is the cost to live is very small, you are not made any happier by spending money on the extras, and you should put this money where it matters to you the most.
Petruchio