How many letters does the top answer for “occurring every six months or during every period of six months” have?

The most common answer is semestral (9 letters).

Crossword clue: occurring every six months or during every period of six months

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

semestral 9 letters wn-def

occurring every six months or during every period of six months

semestrial 10 letters wn-def

occurring every six months or during every period of six months