Save 12 percent to 15 percent of your salary.
Pro: If you're looking for a basic rule of thumb, this is one many experts agree with. "The absolute minimum is 12 percent," Blanchett said. "It's probably closer to 15 percent."

This guideline, like the others here, counts 401(k) contributions coming from both employees and employers. A worker saving 8 percent of her own money reaches the 12 percent goal if her company pitches in 4 percent.

Con: As a rule, it's almost too simple—see the many examples below. The advice is usually based on conservative assumptions about your investment returns. Many experts expect lower returns in the future than over the past few decades, but they could be wrong—predicting the market is notoriously difficult. You could also get lucky and do better than average.

Putting 12 percent to 15 percent in a 401(k) may be unrealistic for workers who have other pressing priorities. When you're young, those include paying down high-interest-rate debt and building up an emergency cash fund. When you're older, a key goal should be paying off your mortgage, Skinner said, because it makes retirement much less expensive.

Base it on how much you make.
Pro: Because of Social Security, lower-income people can get away with saving less, as a percentage of their income, than wealthier Americans. The program, which delivers an average check of $1,360 to 41 million retirees each month, bases its benefits on how much a worker earns over a career, but there are minimum and maximum benefit amounts. Social Security will replace more of a lower-income retiree's income, while higher-income retirees need more savings if they want to live the same lifestyle as when they were working.

Given this difference, Gary Burtless, a senior fellow at the Brookings Institution, offers a revised rule of thumb: A middle-income American, earning roughly $30,000 to $75,000 a year, should start out saving 10 percent to 12 percent of his income. Around age 50, he should reassess. Online calculators, and the Social Security website, can help. "If it looks like they're falling short, they should boost their savings rate to 15 to 20 percent," Burtless said.

People making less than $30,000 can save less than 10 percent to 12 percent; those who make more than $75,000 should save more. "Start at 15 percent and plan on ramping up to 20 percent or more" by the time children graduate from college, Burtless said. "Upper-middle-class earners have a bigger gap to fill."

Con: There are reasons lower-income Americans might need more money saved, while upper-income workers might need less. Well-paid, well-educated workers often find it easier, and more enjoyable, to continue working well past the traditional retirement age. Lower-income workers can have a difficult time working all the way to Social Security's full retirement age, if they lose their jobs or their health deteriorates.

Base it on your job prospects.
Pro: The amount you need to save varies a lot with when you started saving and when you want to retire. If you're a middle-income earner who begins saving at 25 and wants to retire at 62, you need to save 15 percent of your salary, Boston College's Center for Retirement Research calculates. By working to age 70, you need to save just 4 percent. Those eight years of saving, rather than spending, make a huge difference.

But many workers get pushed into retirement by layoffs. And some jobs in some industries are much more volatile than others. "Investment bankers should save a lot, because they could be out of a job next week," said Anthony Webb, research director at the New School for Social Research's Schwartz Center for Economic Policy Analysis. "So should construction workers, whose earnings peak at relatively young ages." A doctor can make a generous salary for decades.