Grooming for such long periods would definitely have me wondering whether some sort of allergy is involved, whether to an insect, food, or something environmental. You're reasonably certain it's not fleas, and his food hasn't changed (although I guess a food allergy can develop at any time). Has anything in the environment changed, like a cleaning product, laundry detergent, etc? My cat was allergic to a carpet deodorizing powder that I had started to use and licked the fur off the base of her tail. Once I stopped using it, she stopped and the fur grew back.
It could also be caused by stress or boredom, and it can become a habit that's impossible to break. My other cat began overgrooming due to stress, and it continued until she passed away. It wasn't harmful, but it certainly wasn't very attractive...Is there anything that could be stressing Marley? The presence of a new kitty outdoors, or something new in the house?