Anywhere from 12-20% depending on service.
Generally speaking our Disney buffet servers haven't been as attentive to us, or cleared our plates as quickly as servers at local buffets.
The exception to this was Gulliver's in 2004. The server here was FANTASTIC!! Fun, friendly, efficient, and everything Disney all wrapped into one person.
We've eaten at Chef Mickey three times and Crystal Palace four times. We generally wait with dirty plates stacked high for most of the meal, have to ask for refills and are just pointed at the buffet when we get there. Even attempts to chat or be friendly with the server are brushed off. We've never had the wonderful experience that others have with servers letting them know when characters are coming, etc.
That rant being over, we'll be tipping the mandatory 18% when we go in March, but if service is bad enough I'll speak to a manager to let them know that there is a problem with service.