we took the bus from paris to mont st. michel (2.5 hrs with 30 min rest stop). spent 3 hrs at the mont st michel (including lunch), we also went up the abbey. took the bus again back to paris (2.5 hrs with 30 min rest stop). overall the place is very nice and a must see.
but the time we spent there is very limited because of the travel time.
i think its best to spend the whole day (excluding travel time) at mont st michel so you dont rush the sceneries.
we did not get the guided tour option but the guide that was with us in the bus, here name is ROSEO, she explained the itinerary plus a little about the place during the bus ride.
she explained it in english and spanish.
will definitely book again.