I think it all depends on which restaurant you are eating at.
For instance, our meal at the Brown Derby lasted 2 hours +. And yes, it felt like a WHILE to me. (Don't recommend it by the way, it's way too expensive for the amount of food you get).
The buffets don't usually take more than 60-70 minutes because the food is ready. Some buffets only took us about 45 minutes because the food was nothing we were very excited about (Hollywood & Vine). We were at Chef Mickey's a while though because we couldn't stop eating. We thought the dinner buffet was fantastic-- probably the best, if not the best, meal I've had in all of my trips to WDW. Chef Mickey's was a blast.