95% of the buses are lift equipped. The longest you might wait would be 20 minutes but the drivers often radio that there is a wheelchair waiting if they aren't lift equipped so the bus gets there faster.
Our experience has been the same as lisapooh. We've been coming to WDW for 12 years with our daughter who uses a wheelchair. We occasionally have had to wait extra for a wheelchair bus, especially at closing time at DD. But the drivers and dispatchers have done a good job of sending a lift bus for us when that happens.