At 24fps, a 2h movie has 172800 frames. 21,970,310 M1 seconds or 8.5 M1 months. Which is less than I was expecting. The rendering seems to scale linearly per core too.
Presumably bad math or a lot more rendering complexity for the pixar super computer deploys?
1) that forest isn't near the complexity of most major feature films. It's great, but it's got a ton of instancing, it's not got very complex shaders, it's not got very complex lighting. It's a good benchmark scene, but it's not approaching the level of scenes in most feature films.
2) You're assuming a single render happens per frame. Every scene in a movie is rendered several times. Potentially every single time a new hand touches it, and definitely multiple times as lighters iterate on it. On average I think a single shot must be rendered about 40-50 times or more in a large production. At least 10 times per each major department.
3) Pixar and other studios don't have really extraordinary computers. They're usually run of the mill HP or Dell workstation specced machines with a dual Xeon and tons of RAM.
4) static scenes with diffuse lighting are fast to render. Add in motion and things get tricky. Do you want motion blur in render (as opposed to composited in via motion vectors)? Well now you're sampling multiple frames. What about depth of field? Way more samples to resolve properly. Now add in more lights, and you need more samples per light to resolve a noiseless render.
Actually, a forest with occluding foliage geometry is a quite difficult light transport situation: it's very difficult for next event estimation to work efficiently, so therefore most of the lighting is indirect and therefore is quite noisy, as it's difficult to find the environment / IBL light for the sun.
Portal guiding and NEE guiding can help a bit, but it can still be quite slow.
But you can denoise a sequence which is similar.