The phrase "many moons" refers to a long period of time, typically years. Some antonyms for this phrase could be "brief moment," "short time," or "instant." These words suggest a brief amount of time rather than a lengthy one. Other antonyms include "single day," "one sitting," or "quick flash," which further emphasize the brief nature of the time frame. Alternatively, antonyms that suggest a limited quantity might include "few moons," "scant amount," or "not much at all." These words convey the idea of a small or limited number of something, which is the opposite of the idea behind "many moons.