How many letters does the top answer for “a period of fourteen consecutive days” have?

The most common answer is fortnight (9 letters).

Crossword clue: a period of fourteen consecutive days

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

fortnight 9 letters wn-def

a period of fourteen consecutive days

any period of seven consecutive days 50% similar → hebdomad, week