At a time when hospitals seem to change CEOs almost as frequently as bed linens, the 27-year tenure of Bethesda Memorial Hospital's approachable CEO, Robert B. Hill, is more than a little unusual. Nationally, the median tenure for a hospital CEO was 3.6 years, according to the most recent survey by the American College of Healthcare Executives, done in 2005. "I am only the third CEO in the 52-year history of the hospital," Hill said. Hill announced plans to retire in February, when his longtime No. 2, Chief Operating Officer Roger L. Kirk, will succeed him.