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

The most common answer is centennially (12 letters).

Crossword clue: once in a hundred years.

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

centennially 12 letters web-def

every hundred years; once in a century

every hundred years; once in a century 60% 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 years. 50% similar → biennially