Evensky's firm has long preached a "five-year mantra" to avoid having to sell investments during market downturns. (Basically, don't invest unless you can hold your investments at least five years.) In addition, as part of a retiree's overall portfolio, he recommends keeping two years' worth of living expenses in short-term money market and fixed-income instruments. The purpose is to avoid disturbing other investments during extended downturns.

This strategy "is working far better than any other alternative I am aware of" during recent market tumbles, Evensky said. With these safeguards in place and as a "very general rule," Evensky agrees with common research that suggests you can spend 4 percent of your retirement savings the first year in retirement, increasing the amount by 3 percent a year for inflation and maintaining an overall portfolio split 60-40 between stocks and fixed-income investments.

So, for every $100,000 you have saved, you can spend $4,000 the first year in retirement.

If you are willing to reduce withdrawals in later years, you may start by withdrawing 5 percent the first year, Evensky said. Of course, you need to factor in taxes and inflation (withdrawals from fully deductible IRAs would all be taxable, so you don't get to spend all the money on yourself).

As to how long the money needs to last, "from a financial perspective, the risk is not dying too soon but living 'too long,'" Evensky said. As a "minimum standard," he said, his firm assumes that clients will live as long as 70 percent of people their gender and age do.

For a 61-year-old non-smoking man, that would be until age 90, and for his 59-year-old non-smoking wife, until 93, for example. For those with long-living parents and grandparents, it would be longer.