How many letters does the top answer for “occurring once a month” have?

The most common answer is monthly (7 letters).

Crossword clue: occurring once a month

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

monthly 7 letters wn-def

a periodical that is published every month (or 12 issues per year)

occurring once in a month; monthly. 75% similar → mensal

occurring twice a month 50% similar → semimonthly

once in two months. 50% similar → bimonthly