How long does it take to get through the whole exhibit (s)?

As long as you want to spend on each exhibit. I would say two hours would be a good guesstimate.

Tack on another hour if it is during the summer and you go to the village. Any day there is big programming can also take longer because most talks/tours can be from around 30mins-1hr. And on days like House Divided (Civil War Reenactment) where the museum is crowded and the events happen all day, things can take well past 2 hours

I homeschool my child so we spent a little longer reading and discussing each part of the museum.