Many stories have circulated claiming the longest anyone has stayed in an ultra-quiet anechoic chamber is 45 minutes, the reason being any longer would drive you insane. To me this sounded like unsubstantiated rubbish, like the claim the Great Wall is the only manmade structure visible from space. So I put my own psyche on the line, subjecting myself to over an hour of the most intense quiet on Earth. No, this was not THE quietest room on Earth (-9dB) but it is one of the quietest, and the truth is once you put a person inside, they are by far the loudest thing in there so the sound rating of the room is irrelevant.
I was not surprised to find that I could stay in there for as long as I liked and feel perfectly fine. What was surprising is that my heartbeat was audible. You can hear it on the sound recording. Now I wasn't consciously aware of the sound of my heart while in the room, but I was more aware of the feeling of it beating.