。
How long is the flight?
Literal
Flight-time [topic-は] how-much [copula-ですか].
飛行時間 (ひこうじかん, 'flight time') is a compound noun. どのくらい and どれくらい are interchangeable expressions meaning 'how much/how long' — both are common in everyday speech.
。
How long is the flight?