I guess what prompted the question is having read so many WDW buffet reviews that included "after they brought my drinks I never saw him/her again".
I think a majority of people saying this are just not aware of their surroundings. They are all excited at disney, talking with the wife/husband/kids, enjoying the food, etc. My ex was a disney buffet server, and she ran her *** off everynight. I don't think a 5 minute period would go by that her tables couldn't see her. She was always either greeting a table, getting refills, taking dirty dishes to the kitchen, cleaning a table or printing a check. Now if a party of 12 wanted 5 separate checks, or she just got their drinks orders, she would be away for 3-4 minutes, but thats it. A lot of people are enjoying their family time, so she wouldn't bug them every 5 minutes asking if everything is OK, or if they needed something. She would simply walk past the table, and take a glance. If dishes were dirty, she would remove them. If a glass was empty, she would to get a fresh coke. If she walked by, and everyone is enjoying food, has drinks, is happy, there wasn't a need to interrupt them.
Now I know someone is going to jump down my back with a disney buffet horror story about how they NEVER saw their server, waited 14 minutes for a refill, etc, but that would be an extreme, rare event, if even true. People tend to stretch the truth when angry/hungry. I use to serve at apple bees while in college. Their computer system monitors the time period from when its rang in, until its ready to leave the kitchen. Many, MANY times I would hear, "Um, I've been waiting like 20-25 minutes for my dinner." I would go check, and see its been 14 since it was rang it.
To the original question, yes, 15-20% is standard