I don’t believe the average is 24 hours of wall clock. I do think average render times have increased a bit over time, but FWIW, I think the average render time needs to be “overnight”. The shot just needs to rendered before dailies in the morning. If it takes longer than maybe 6-8 hours, it risks not being done by the next day, and that means each iteration with the director takes two days instead of one. There is significant pressure to avoid that, so when shots don’t finish overnight, people generally start optimizing.
When I was doing CG production shot work ~15 years ago, there were occasionally shots that ran 24 hours, but the average was more like 3 or 4 hours. The shots that took 24 hours or more usually caused people to investigate whether something was wrong.
I worked on one such shot that was taking more than 24 hours. A scene in the film Madagascar where the boat blows a horn and all the trees on the island blow over. The trees and plants were modeled for close-ups, including flowers with stamens and pistils, but the shot was a view of the whole island. One of my co-workers wrote a pre-render filter with only a few lines of code, to check if pieces of the geometry were smaller than a pixel, and if so just discard them. IIRC, render times immediately dropped from 24 hours to 8 hours.