While I don’t have any ground to take issue with the rest of your post, you start out with an inaccuracy that belies an unfamiliarity with adult video.
I’ve screened thousands of adult VHS and DVD titles, and with very few exceptions, the scenes are 20 minutes long. Sometimes maybe 19, others maybe 21, but it’s like an unwritten rule: 20 minutes per scene. There are some directors who intentionally break this rule (Rocco) and solo scenes or compilation tapes or cable TV edits may often be shorter due to editing, but I’ve even seen comp tapes loop parts of scenes to bring them up to the 20 minute mark.
Why 20, I don’t know. Perhaps it’s what they think the average wank session lasts, or maybe it was someone’s idea of what a typical sexual encounter should last, and other directors/editors just mimicked the habit.
But anyone who consumes adult video has learned the “20 minute rule,” especially users of VHS decks.
Now, that said, it’s only the lowest of the low budget “Gonzo” tapes where you can hear the director giving orders. Some say that’s part of the definition of “gonzo” – that part of the depiction is the acknowlegement of the presence of the camera. I call BS on that… it’s just low budget, all sloppy camerawork and hack editing.
Even high-volume schlock houses like Anabolic or New Sensations will edit out obvious direction. Sometimes, you can see the “talent” looking off camera at signals from the director or crew, but that’s about it.
As for the specific shooting ratio, videotape is cheap, so economy of media makes 20:1 a reasonable guess. However, time is not cheap, so I would expect that they are closer to 5:1. And the few articles and interviews I’ve read about the process tend to agree with it.
Forgive me for not having a cite, but I’m at the office… 