Americans used to be better at taking time to take off. During the 1980s and 1990s, U.S. workers burned through 20 days of vacation per year on average, depending on seniority.

Currently, it's about 17 days.

Still, more than half of American workers—around 52 percent—aren't even using all the vacation time they do have, according to Project: Time Off, an initiative of The U.S. Travel Association to track and research vacation trends.

And if you're not taking your vacation time, you'll pay for it one way or another.

"On the employee side, if they're forfeiting that time, they're basically volunteering that time," Katie Denis, Project: Time Off's vice president, told CNBC in a recent interview.

Denis said that more than 200 million vacation days were left on the table in 2017, and the lost value of those untapped benefits was "$62 billion last year alone."

For each worker, that "donation" to their employer of untapped paid time comes out to $604. Denis said that unused vacation by employees also hurts employers.

"On the company side, they're carrying that vacation on their books," Denis told CNBC. "If it's being rolled over, eventually they're going to have to pay that out."

Yet companies benefit when their workers use their time to get away from work for a while, Denis added.

"What we find is employees who take more vacation and specifically those how travel and get a true break, is that they're more creative, more productive they're happier with their health and wellness but also with their job and company, so this is a massive retention angle as well," she told CNBC.

In this year's Project: Time Off report, they found nearly a quarter of Americans, 24 percent, have gone a year without a vacation. Meanwhile, 12 percent say it's been three years or more since they took time away for a trip. So what's stopping them from getting away?

61 percent of employees surveyed say it's the "fear of looking replaceable," while 56 percent insist their "workload is too heavy."

As for which generation is better at using all their vacation days, Denis said millennials are the worst. That age group takes 14.5 days of vacation, Project: Time Off's data showed, while Generation X takes 17.9 and Baby Boomers take 19.8, the organization's information showed.

That goes against the stereotype that workers in their 20's to mid-30's "are entitled, they're spoiled, they're lazy, they take everything they're given," Denis said. "Our research finds the exact opposite."

A number of independent surveys have underscored that millennials are preoccupied by a host of worries, including heavy debt loads and career fulfillment.

"Sometimes we forget that most of that generation graduated into a really difficult economy," Denis said. When they land steady jobs, millennials "really want to hold onto them."

Denis acknowledged that millennials often have fewer vacation days, since they're just beginning their careers. Still, "they are very concerned about looking replaceable." And their work style differs from older generations too, she added.

"Face time is not really their currency. It's not a matter of being the first car in the office parking lot," Denis said. "But it's being the first person to respond to an email. While they show up it's a different way of doing that."

On the Money airs on CNBC Saturday at 5:30 am ET, or check listings for air times in local markets.