which one is correct and why "我看书看很长时间” or "我很长时间看书”
"看书“ is detachable word， that means we need to put temporal words(such as 很长时间）into the middle of "看书”。Detachable word can be collocated with many kinds of word or phrase and the collocation with quantitative phrase is one of the most common occurrences。
So the correct answer is "我看了很长时间书" or "我看书看了很长时间”。

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