If you haven't already gone, I would spend 3 hours or so. There is very much info & well laid out. (3 floors total )
I would agree with between two and four hours, depending on your level of interest. I and my ten year old son spent about three hours altogether, between the museum and the memorial.
We love state history museums, so we were there 3-4 hours without kids. I could see getting a good experience in around 2 if you were in a hurry.
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