How many letters does the top answer for “quickly and without warning” have?

The most common answer is abruptly (8 letters).

Crossword clue: quickly and without warning

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

abruptly 8 letters wn-def

quickly and without warning

short 5 letters wn-def

primarily temporal sense; indicating or being or seeming to be limited in duration

without warning or announcement 50% similar → unannounced, unheralded, unpredicted