To step off the corporate treadmill in your 50s or early 60s and maintain anything close to your standard of living, you need a seriously big retirement kitty.
How serious? You'll likely need assets worth 10 to 16 times your salary by the time you leave your job. A 45-year-old making $120,000 who hopes to retire at age 60, say, should already have nearly $700,000 set aside. (See the Retire Early calculator.)
You can get by with less if you'll have other sources of income. If that same 45-year-old has a typical old-fashioned check-a-month pension, for example, he might need only $432,000 in savings to be on track. If you expect to hold down a scaled-back job for your first decade of retirement, you can also get by with less.
Still, your target is a big number, and to reach it you'll have to save diligently, invest aggressively, and keep taxes and expenses from eroding your returns.