Spine plays back animations at 30fps. So if you have an animation that is 80 frames long and you export all 80 frames at 24 fps the animation would be longer than what you had in Spine.
80 frames at 30 fps is 2.6 seconds.
80 frames at 24 fps is 3.3 seconds.
64 frames at 30 fps is 2.1 seconds.
64 frames at 24 fps is 2.6 seconds.
You can do the calculation yourself like this: TotalFrames/30ExportFramerate
In this case 80/3024 = 64
I hope it helps 🙂