I'd say if you are a real history buff like me, three full days is good enough, at minimum. I didn't use a guide, but I am very knowledgeable in general so I know all the basics. I just rented a car in Caen and drove around the region, and essentially parallel to the coast. Take the train from Paris to Caen. Car rental in Caen is right at the station. I loved driving there. Easy, no problem adapting. They drive on same side of road as us, just different signage of course, and lots of roundabouts, but a normal American driver will have no problem if you have good GPS.
Obviously a good guide would be golden and valuable. I'm just saying it's fine to do it without one if you are knowledgeable enough.
There are so many museums in the region, like 15-20. You could never do them all unless you had a couple weeks. It seems as if so many people/groups there decide to open a museum as a way of making money so they are all over the place, and usually with slightly different themes or focuses. So plan out which interest you. That's why you don't need a guide if you want to hit museums.
In 3.5 days I was able to hit: the American cemetery, Point Du Hoc, Pegasus Bridge, Caen museum and underground site, Arromanches (fantastic museum there...something dubarquement) with amazing scale models of the logistics of floating piers and such, and Arromanches 360 film experience), the German cemetery, all 5 beaches, including the Canadian museum and Canada House area on Juno, the Canadian cemetery in a different area, the Band of Brothers monument outside Brecourt (you can't go on property as it's private, the Dick Winters statue, Utah Beach museum (really good) and the best museum of all in my opinion, the airborne museum St. Mare Eglise, plus numerous coastal artillery and fortification remnants. So you can do a lot in 3 days, especially in Summer when the Sun doesn't go down there until about 10pm. I was there during May.