Very simple metric that has served me for years: If your workload doubles when a coworker or counterpart takes their PTO—which is their right as a human being—your company is understaffed.
Working twice as hard to cover your coworkers is inappropriate and a clear-cut sign of poor management. Don't work harder, don't work faster, work at your pace. I'm slammed with about triple my usual workload this week because the other guy who does my same job is off all week. Just because he took his time off on a busy week means nothing—he deserves it. The real problem is the company being understaffed because profits and cutting costs are the priority—not you, not me, not any of us.
For those of us working double today: Tell your manager when you are spread too thin, and don't pin the blame on anything or anyone but management. It's their fuckup, not anyone else's. For those of us needing a break: Take your time off when you want to—that is why it's there. No guilt, no shame, nothing but the enjoyment of your leisure and the restoration of your energy.
Fuck bosses. Organize and unionize. The US is a shithole.