Then I started to wonder about someone exclusively listening and reading for say 3-5 years straight, no speaking at all, say 8 hours + per day, to the point where literally nothing they read or listen to is the slightest problem to them.
I've been in a similar situation in the past, although not exactly the same. Notice a couple of things:
- Real conversation is not the same as listening to recorded audio. Someone who has never spoken would not be so used to understanding real conversation, just canned audio. Even understanding may take some getting used to. Demanding recorded video/audio (such as films) is typically harder than face-to-face conversation but there are, nevertheless, some factors that may hamper that person's understanding: noise, anxiety from having to reply, etc.
- If you're level is so high, making yourself be understood is really not all that hard, so chances are you can get your message across but that doesn't mean that you'll be "fluent". A big factor is your expectations. That hypothetical person
Probably, the person you're thinkng about would be in a vey similar position to someone who reached a good level in a language years ago but haven't brushed up on it for a long time: they'll understand most of what's said with some misunderstandings here and there and will be able to explain themselves with lots of hesitations to find the right word, many convoluted expressions and lots of grammatical mistakes that they themselves will identify as such.
As Steve explains, one week of immersion would be enough to take you to a nice (not perfect) speaking level.
My own experience: I learned French and English through a few classes and a lot of lonely work when I was young. I had spoken a bit in a class context but mostly I had read, read and read and I eventually purchased a short-wave radio for practice. It was a kind of pre-internet LIngq regime for the most part. Later, I had the chance to visit France and Britain. Again, I was not in the exact same postion as your hypothetical case. For example, I hadn't watched films in the language (radio is different) and my grasp on slang was so-so. I suppose you can get more and better listening practice nowadays. However, I think the end result may be not so dissimilar.