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It took her over three months to finish reading the magazine.
Literal
She [topic-は] magazine [object-を] finish-reading [for-のに] 3-months more-than took.
A clean illustration of two productive structures. The compound auxiliary ~終える attaches to a verb stem to mean 'finish doing X' — 読み終える ('finish reading'), 食べ終える ('finish eating'). The nominalizer + のに here is the purposive 'for the purpose of': 読み終えるのに ('for finishing'). The whole frame [time/effort] + のに + かかる expresses 'it takes X to do Y' — a hugely useful construction for talking about durations of effort. 以上 ('more than, above') marks an inclusive 'at least.'