After the Retirement-Calculator-Extravaganza featured in the p0ast below (read that one first), I had the brilliant idea to consider early retirement. If I can live off around $30,000 a year and don't have to worry about leaving money to my kids:
If I contribute 6%, I'd have to work until I was 67. (There's a little surplus, so actually 66.6)
If I put in 10%, I'd have to work til 61.6. Getting better.
If I can bump it up to 15%, it's down to 57.6. Not too shabby!
And at that crazy 20%, it cuts it down to 54.7. Not as many years as I figured it would shave off, considering how much of your salary would be going into your retirement account. Then again it's an extra 11 years of not working.