How close to the real thing?

Depends on the music, setting and venue. I was in a very upscale hotel recently that had a jazz trio, bass, sax & piano. They were playing music that I was very familiar with and I was feeling quite good about how well my systems sounds compared to the live artist. Instrument placement, tonal balance is very good, I would rate literally 90%. When I go to a live orchestra. My system cannot convey, the real depth and stage presence that a live orchestra conveys over the large stage. Maybe I don't have good enough recordings on classical, I must admit, I am weak in classical recording and my jazz selection is quite good but my number drops profoundly.

If memory serves, the late Peter Walker (Quad) answered to such a question- about 15%.

I disagree with the previous answers.
From the starting point of recorded music.. The wax cylinder.. the sound THEN was perhaps at least at 30% in reality. (consider it an exponential curve)
By the 50's we were at least up to 80% or better.
Now i would say state of the art os up near 90% to 95% minimum.
Of course with an exponential curve, the closer we get to 'real' the harder it is to get better!
And with a curve going to infinity (true in every way to perfect reproduction) we probably will NEVER get to !00%.. but we are a Hell of a lot closer than 5%!
IMO that sort of answer is just to elicite consternation/controversy, and has no merit, or is taken out of context.
And of course as we get better, the gaps in what is left to achieve become more apparent, and perhaps the desire to get those last little itty bitty bits might make one say we are so very far away from 'perfect sound forever" LOL.

I guess it is all perspective, I can tell you that on the small jazz ensemble that I told above. Instrument placement is outstanding as is height and width, the realism of the instruments also right on. I have had more comments than I care to mention along the lines of "wow, it couldn't get any better without having the artist with you"... There is no way 5% draws those comments.... Of course..... being perspective?
Good Listening, Tim

Elizabeths answer +1 - I'll go as far to say BOSE is better than 5%. OTOH if the recording is bad - well it may very well be less than 75% - That's my opinion