A day each the first time. Growing up there, I visited the village maybe 10 times, a whole day each, over 15 years, and have been back for a whole day another 5 times in the past 30 years. I've been to the museum at least five times, some of those being guided "theme" visits into areas not usually seen by one-time visitors with just a day. You cannot see everything in the open collection in one day, but you can likely have a quick look at everything really of interest to you.
Both parts of The Henry Ford are constantly changing, and special events are frequent during the summer season, special exhibits from time to time. I still have family in the area, and some of them maintain memberships and visit both places several times a year.