How many letters does the top answer for “relating to or extending over a relatively long time” have?

The most common answer is semipermanent (13 letters).

Crossword clue: relating to or extending over a relatively long time

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

semipermanent 13 letters wn-def

relating to or extending over a relatively long time

relating to time long past 50% similar → olden