If you search for any video or article that teaches the past perfect continuous tense, you'll surely come across an example such as:
"He had been throwing rocks at her window for five minutes before she finally came out on the balcony."
If you didn't know the part about five minutes, it would be only natural to ask, "How long had he been throwing rocks at the window before she finally came out?"
If I understand it correctly, it would be wrong to say:
"He was throwing rocks at her window for five minutes before she finally came out on the balcony."
(As the past continuous tense is natural for particular moments. For example, "...at 11:35", "...all day long", "...when she opened the balcony door.", etc.)
So I draw a conclusion that it would be equally wrong to say: "How long was he throwing rocks at her window before she finally came out on the balcony?"
However, here's a little dialogue in which I heard a well-educated American use past continuous where I would expect to see past perfect continuous:
Chuck: How long was I unconscious?
Ernesto: I'd say, maybe, about a minute, you were kind of in and out.
Chuck: And how long was I lying on the floor before the ambulance arrived?
Ernesto: About 10 minutes, I think, but I didn't look at the clock.
Better Call Saul
So, should I view that usage as a minor aberration in speech? A slip of the tongue, maybe? Or am I completely misconstruing the whole usage?
Update 1:
Some of you have suggested that it's idiomatic and grammatically correct to say "He was throwing rocks at her window for five minutes before she finally came out on the balcony." and "How long was he throwing rocks at her window before she finally came out on the balcony?" But that suggestion seems to be in conflict with what Michael Swan's authoritative "Practical English Usage" says:
How long? past perfect progressive, not past progressive
We use a past perfect progressive, not a past progressive, to say how long something had been happening up to a past moment.
We’d been walking since sunrise, and we were hungry, (not We were walking since sunrise...)
When she arrived, she had been travelling for twenty hours, (not ...she was travelling...)
So who's right?
Update 2:
People keep suggesting another thread as a possible answer to my question. I'd like to assure those people that that thread doesn't answer it, as my question is not about reported speech. I'm fully aware of the optional back-shift in reported speech, thank you very much.
Another, I'd like to thank those of you who pointed out the difference in emphasis between the PPC and PC in the construction at hand. I wonder if that difference has any importance in light of the structure we're examining.