Similar to millicenton, microcenton was used exclusively in an early episode, "Lost Planet of the Gods, Part II". In it, Starbuck told Apollo he was "switching off of you for a microcenton to scan ahead." If you consider that the early use of the centon in the pilot episode was something closer to a week, the microcenton would be somewhere in the neighborhood of a half second, or one millionth of a week.
The term was superceded by micron.
See also: Units of Measure (TOS)