pronunciation - Do native speakers think of prolonged vowels as one long vowel, or two vowel sounds following each other?

In a native speaker's mental model, are long vowel sounds - for example the おう in 「教室」 the ああ in 「唐揚げ」, the えい in 「先生」 or just 「いい」 (I chose these examples quite deliberately, since they represent fairly different "categories" of long vowels; please consider them all) - a long vowel sound or two vowel sounds following each other? In other words, when they say a long vowel, are they deliberately saying one long vowel sound or two of them directly following each other?

Does it perhaps depend on which vowel it is? Does it depend on the word? Perhaps neither, or something else entirely? This is something I've been wondering for a while now, and I feel knowing the answer would help my latent understanding of the language and pronunciation.

I realize there's probably no definite answer, at least not without conducting some scientific research, but having the input of some native speakers or someone who's talked to native speakers about this would be invaluable.