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.