[user104658]

It's talking about a very specific circumstance though - people with critical injuries flatlining on the operating table (and with the critical injuries not including head injury, because then you'd be out cold) and also I suppose sudden heart failure. It's not actually a very common way to go. Most people slip in and out over at least a few days... and in that case, there probably are moments of lucid thought where you realise "I'm not getting better am I", but you're most likely to be unconscious already when the body actually stops completely.

Either way... it would be seconds or at most minutes of awareness. Someone in the comments is talking about hearing being present for up to 20 minutes, which is nonsense. I mean... have someone press on your carotid artery and see what happens. You will pass out cold in under ten seconds, no awareness at all. You are alive, your brain is still functioning, but you're certainly not going to be aware of your surroundings or anything else. And that's the same thing that happens after your heart stops; no blood gets to the brain. So realistically, 10 seconds after your heart stops, there might still be measurable brain activity but the person would be "passed out", very unlikely that there's any conscious thought going on.