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

The most common answer is biennially (10 letters).

Crossword clue: once in two years.

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

biennially 10 letters web-def

every two years

once in a hundred years. 50% similar → centennially

once in four years. 50% similar → quadrennially

once in seven years. 50% similar → septennially

once in six years. 50% similar → sexennially

once in three years. 50% similar → triennially

once in two months. 50% similar → bimonthly