Just a neat equation I found today while counting 3D video frames.
I estimated a 3D frame in a series of them in a particular animation I was creating would take 20 seconds each to render and I had 180 frames (30 frames per second) thus my clip is six seconds long.
So I multiplies 20 seconds by the total number of frames and reached the number 3600. I divided that number by sixty (the number of minutes in an hour) and the result told me it would take roughly an hour to render.
The variable in this case is how complex the scene is because every frame can render at different speeds just as some animations are complex and some are simple, thus the 20 seconds is an average of this animation.
Also consider 180 ÷ 3 is also 60, 1/3 frame per minute times 180
This equation kind of twists the mind in a peculiar way.