How many letters does the top answer for “the state of not existing” have?

The most common answer is nonexistence (12 letters).

Crossword clue: the state of not existing

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

nonexistence 12 letters wn-def

the state of not existing

the state of not being 50% similar → nonbeing

the state of not coalescing. 50% similar → incoalescence

the state or fact of existing 50% similar → being, beingness, existence