Remember that you may have other sources of retirement income, such as a pension or Social Security. By the way, most people receive higher benefits by waiting until they're 70 to start receiving Social Security payments, but if you have reason to think your retirement will be half as long, you may prefer to take the benefits as early possible. The Social Security website (ssa.gov) has a useful break-even calculator to help you figure out the best time for you to start receiving benefits.
Obviously, it's critical to know how long your money will last, but it's just as important to understand how much money you'll need in retirement. This is pretty easy to do if you're retired (you know how much you're spending). If you're not out of the work force, your take-home pay is a reasonable (and conservative) proxy for your retirement income need.
Ultimately, the key number is the difference between your needs and the income your assets can generate, along with Social Security and pensions. If it's positive, you're fine! If it's negative, you're going to have to change some things or you may risk running out of money early.
Finally, these guidelines are based on the assumption that you will have a 7 percent long-term return from a portfolio composed of approximately 60 percent stocks and 40 percent fixed income and cash. The potential for growth from stocks is crucial; otherwise, you face a higher risk of not keeping pace with inflation.
A good way to manage your portfolio in retirement is to have a year's worth of money in cash, then one-year and two-year CDs for the second and third years, and then the rest in a diversified selection of stocks and bonds. After year one, cash the one-year CD and that money becomes your living expenses for year two, and you'd buy a new two-year CD. That way, you'll have the money you need for the next three years easily accessible.
Hope this is helpful. Best of luck!