「いつ戻りますか」「天候次第です」

「」「」

"When will you be back?" "It depends on the weather."

Literal

"When return [polite-question-ますか]?" "Weather dependent-[copula-です]"

次第 (しだい) is a versatile noun/suffix meaning 'depending on X,' 'as soon as X,' or 'by X's will,' depending on context. Here, 天候次第 'weather-dependent' functions as a nominal predicate: '[it] is weather-dependent.' 次第 attached to nouns like this is the standard way to express 'depends on.'