What is the shortest time you can animate and loop without it looking like a loop?

I’m guessing it all depends on the total length of the loop itself and the complexity of the shapes/animation. But on average, lets say for a 2 second loop, at 24 ftps, how many frames can I animate and duplicate and not have the human eye detect it was the same thing repeated.

Ex. this is done all the time in animation (at least on tv) in scenes where someone is running, driving and the background is a bunch of trees/bushes/building just looping. I notice that some are done well so that the second you’re about to recognize and match the previous frames to the upcoming ones, the shot ends.