From my own experiences, unless you see the waiting time change in front of you, 15-20 mins usually turns out to be a lot longer than 15-20 mins. (Except in early morning or late at night) There would be many many people who would jump to a line that say 15-20 mins wait, no matter if they wanted to ride it or not. The wait could be 15-20 when it was first displayed, but quickly increase after that. At the same time, many non-headliner rides that listed a super long wait would have a much shorter shorter actual wait. There were occasions when Under the Sea was almost a walk on at 45 mins listed and 30 mins wait when it listed as 10 mins outside.