Folks, just go read the FIRE (Financially Independent / Retire Early) blogs:
http://www.mrmoneymustache.com,
http://jlcollinsnh.com, lots of others), or listen to the Choose FI podcast.
The Trinity University retirement study showed that you need a stock and bond portfolio of
25x your annual expenses ("4% safe withdrawal rate") to successfully retire for 30 years or more. The researched scenarios included all the bad investment periods of the last century, including the Great Depression and the 60's-70's, and often the portfolio would have actually grown in size. The 4% withdrawals in the study were adjusted for inflation, by the way. Check out this
article for more details.
So first of all, save & invest as much as you can. The most vital thing is to control your expenses and don't inflate your lifestyle with every pay raise. You don't need a new BMW, you don't need a McMansion, and you don't need $12 in coffee and pastries every day. Read those blogs and you will find examples of early retired people living on as little as $25k ($0.63M portfolio) a year, in very nice places (Colorado, Vermont, Georgia, etc.). There are people living really large on $40k ($1M portfolio), or $80k ($2M portfolio) a year.
Add in Social Security income, a pension, a side business, or even a small part time job and you significantly lower the required size of the portfolio. A portfolio of $3M or $5M would be very nice, but it's not required.