I think it really depends on you and your style of eating. Whether you take your time or not.
For non character meals (dinner time) we are pretty much in and out within and hour. We don't linger, but we don't rush either.
However there have been some longer times depending on the place and or server.
Example: Ohana gets very slow the later in the evening it gets. At least this is our experiences. Anything past 6:30-7pm it can take up to 2 hours. This annoys me, I love that restaurant and the food, but it kills me to take that long to eat a meal. (needless to say we don't do signatures)
Character buffets aren't much different, except maybe adding 15mins to 1/2 hour more depending on where the characters are when you were seated.
Most do the restaurant in 45 mins...so you should be good with them no more than 1 & 1/2.
Hope this helps.