There are several synonyms for the phrase "of long duration." One commonly used synonym is "prolonged." This word refers to something that lasts for an extended period of time and may connote a sense of endurance or perseverance. Another synonym is "lengthy," which implies that something is lengthy or drawn out. "Extended" is another synonym that conveys the idea of stretching beyond a normal or expected time frame. Other possible synonyms include "sustained," "continual," "persistent," and "protracted," which all emphasize the idea of something lasting for a significant period of time. Each of these words can be used to describe events, processes, or experiences that take a long time to occur or develop.