Doesn't the sentence need a preposition "for" at the end?
Answer from a Native speaker
Although the sentence is fine without it, you can add "for" at the end without changing the meaning of the sentence! Speakers sometimes drop prepositions like "for" at the end of a sentence or question, like in this video. Ex: How long have you been waiting [for]? Ex: How many years have you been playing guitar for?
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