The big difference between hoarders and collectors: One of them appreciates what they’re piling up. The U.S. stands alone among developed countries by not mandating vacation time. Of those who get vacation time, four in 10 Americans stockpile them, failing to take all the days they’re offered. Those stay-at-work Americans leave an average of 8.1 days unused, according to a 2014 Oxford Economics analysis. That’s about 429 million unused days per year.