。
She waited hours for him.
Literal
She [topic-は] how-many-hours-of duration him [object-を] waited.
A near-cousin of the bare 何時間も pattern but with a different feel: 何時間もの間 explicitly extends the time span as a noun phrase ('a stretch of many hours') by adding の間 ('the duration of'). The stress is more on the elongated waiting than on simple iteration. 待つ as a transitive verb takes its object with を rather than を — a small note for learners who may have learned ~を待つ as a fixed pairing. The slightly more elaborate construction fits writing or narration where the duration is meant to land emotionally.