How many letters does the top answer for “once in four years.” have?

The most common answer is quadrennially (13 letters).

Crossword clue: once in four years.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

quadrennially 13 letters web-def

Once in four years.

a period of four years 50% similar → quadrennium

once in a hundred years. 50% similar → centennially

once in seven years. 50% similar → septennially

once in six years. 50% similar → sexennially

once in three years. 50% similar → triennially

once in two years. 50% similar → biennially