Seasoned traveler here. Have been to France many times, but have yet to visit Mont Saint Michel. Well, this May I'm finally going to set aside a day from my stay in Paris and put up with the long route. I think the train/bus connections are much better now than they were a few years ago when I last visited Paris. According to the SNCF website, there is a train/bus combo (Montparnasse to Mont St. Michel) that takes less than 3 hours each way. I only have time to visit this part of Normandy in case there are those of you who chime in to consider spending more time to see the war sites (I don't have enough time allotted for these sights). And I do realize that I will be visiting when the throngs of tourists are there. I'm ok with that.
My question, really, is how much time should I allot for a day trip here? I want to purchase train tickets now to save money and I want to be sure I'm not spending way too much time there. One schedule has us in Mont Saint Michel by 10:30 and leaving back to Paris at about 6 pm. That would give us about 6 hours. Is that too long?