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She'll often just sit there for hours doing nothing.
Literal
She [topic-は] anything [also-も] not-doing many-hours [also-も] there [at-に] is-sitting [thing-こと] [subject-が] often exists.
Two distinct uses of も sit in this sentence. 何も + negative produces 'nothing'; 何時間も attaches も to a quantity to express 'many hours.' The frame ~ことがよくある describes habitual occurrence ('it often happens that...'), structurally distinct from ~たことがある (past experience) — same こと, very different meaning. Pairing 何もしないで with 何時間も creates a deliberate emptiness, a portrait of inactive endurance.