I would have to completely disagree. It really depends on what is happening in that 1/8th a script. Is it a man sitting solemnly alone in the dark. Or perhaps two luxury cars speeding down a windy road. I wouldn't suggest looking at eighths in units of time. It's all about the scene, the directors vision for that scene and how the cinematographer wants to shoot it. The same 1/8 can take widely different amounts of time depending on how the creatives want to make the scene.
There are a lot of great books, that break this down much better, that have also been around for decades. I'd suggest using those as guides.
Step one: Make a poorly made PSA mocking those who think they know what it means to make visual media. The irony is so thick I could cut with a fucking chainsaw.