If you think the standard recommendation of putting 15% of your paycheck toward retirement is impossible to achieve, get ready for an even bigger hurdle. At least one retirement expert thinks that number should be much higher. Millennials should aim to set aside nearly half of their income for the future, according to Olivia S. Mitchell, professor of insurance/risk management and business economics & public policy, and executive director of Wharton's Pension Research Council at the University of Pennsylvania. If you want to live off even half of your final salary in retirement, you need to save 40% of your income over the next 30 years, she says. That calculation, which is based on academic research from an MIT economist, takes into account a few assumptions. The biggest is that you want to retire at 65. That's not much of a stretch: About a third of millennials say they expect to retire between the ages of 65 and 69, according to a recent T. Rowe Price survey. However, 43% of millennials say they actually expect to retire earlier. Yet about half of millennials are planning to contribute less than 6% of their income to a 401(k) this year, the T. Rowe Price survey found. Only about one in five is currently saving more than 15% of their income. The second major assumption is that investment returns over the next few decades aren't going to match the roughly 10% historical returns Americans have enjoyed previously. "Most people are not told by financial advisors that their future returns will likely be much lower than in the past, and their future taxes will likely be much higher," Mitchell tells CNBC Make It.

Other experts agree. The economists at investing giant Vanguard predict that, over the next 10 years, annual U.S. stock market returns will likely average 3% to 5%. When you factor in inflation — which, luckily, Vanguard predicts will be below 2% — the real rate of return is expected to be under 3%. Morningstar Investment Management predicts an even more meager return: 1.8% over the next 10 years for U.S. stocks, before adjusting for inflation. Meanwhile, perhaps the most pessimistic outlook comes from Boston-based asset management firm GMO, which expects real returns of -3.6% for U.S. large-cap stocks and -1% for small-cap stocks. It's also worth noting that the 40% retirement savings rate calculation does not take into account Social Security's looming shortfall. If Congress does nothing, the agency's funds will only be able to pay out about 80% by 2035. But many experts are betting that reforms will happen.

If you can't increase your savings, you need to change the math