How many days could you easily spend in Normandy?
A week, easily. A couple of days doing Battlebus tours, a day walking around Bayeux, especially for the Saturday market, a couple of days visiting Mont St. Michel, visiting the copper workshops in Villedieu, following the Route du Cidre sampling calvados and pommeau, and a couple more days because you will encounter more things to do simply following the itinerary I outlined above. Normandy is easily our favorite part of France (so far). Assuming you are heading back to Paris, don't miss Les Andelys on your way to Monet's Garden in Giverny.
My husband and I were there in 2006 and spent only two nights, one near the D-Day sights and one in Mont St. Michel. We visited Omaha Beach, the American cemetery, the Caen Peace Museum, the Bayeux Tapestry and Cathedral, and then Mont St. Michel. We easily could have spent another night or two, in order to see the other D-Day spots, more of Bayeux, and just touring the general area. It's beautiful and different from the other areas of France we visited.
Normandy is also our absolute favorite part of France outside Paris. I agree with John's post, we also could easily spend a week there and still feel sad to leave. Just driving around on the country roads exploring is magical. We especially love Bayeux, staying at Hotel D'Argouges, the D-Day beaches, St. Mere Eglise, the American cemetary, all the little villages, and the people...they are wonderful.
We just returned from a 4-day trip to Normandy and definitely could have spent more time there. Mont St Michel and the Battlebus D-day tour were definitely the highlights of the trip for us. I'd also recommend considering Dinan in Brittany (just 40 minutes from Mont St Michel). We stayed one night and very much enjoyed walking the town and the path along the Rance river.
Easily a week but you could hit the highlights in 4 or 5 days. We stayed in Caen and visited the museum and got our bearings. We rented a car for three days and did the landing beaches and Point de Hoc. We also went to Honfleur and Mont St. Michel. Not sure how we missed Bayeax -- I remember passing the town but didn't stop. One day we took a guide (Canadian university student) to the beaches and the Canadian cemetary (there's an American one, too). It was such a moving experience. We had fabulous mussels in the town at "Omaha Beach" and hiked the hills overlooking the beach. They were full days.