It depends a lot on how you do museums. If you just buzz up and down the aisles like you're at a trade show you might be able to do it in a day, but you'll be missing out on one of the best collections tracing the history of technology and how it shapes our lives around. Three day minimum, and if you do the Rouge Factory Tour (HIGHLY recommended) four would be better. A full day at each site, a half day at the Rouge, and the remaining time to catch what you missed the first time through. Try to plan your Rouge tour either first thing in the morning or right after lunch break so that you can see the line in operation. The factory floor part is self-guided on a catwalk above the final assembly area - we found it easy to kill 2 hours just watching that. Great place.
We stayed at the Wayne County Fairgrounds and found it convenient and decent enough - we're normally state park campers but for a destination trip like this it's fine.