What with the economy in so much turmoil, taking a good, long vacation might not be on your to-do list this summer. The demand for luxury vacation properties seems to be fading in places like Grand Lake, on the majestic western side of Rocky Mountain National Park (via BusinessWeek ). But you don't have to plunk down millions to have a good time, right? In fact, all you may really need is a good 10 days, according to research by Dena Ann Dries, a doctoral student in psychology at the University of the Rockies in Colorado Springs (via the Denver Business Journal ). Her research indicates that there are diminishing returns on happiness, more or less, during a vacation---that the mental and professional benefits of getting away from the office peak at about 10 days before they start to decline. "There is a point at which adding vacation days ceases to increase job satisfaction, ceases to impact retention, and ceases to impact professional well-being," according to her study . "Between groups, analysis revealed that participants who took more than three weeks of vacation showed no significant differences in any of the workplace variables with the exception of extreme burnout." Previous research found that 10 to 14 days is likely the optimal length for a vacation.