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."