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How long is the stopover?
Literal
Stopover time [topic-は] how-much is [question-か]?
立ち寄り時間 (stopover time) uses 立ち寄り (stopping by) + 時間 (time). どれくらい (how much/how long) is the standard way to ask about duration or amount.
。
How long is the stopover?