I have worked in a few different anechoic RF chambers, which are also very quiet. The wedges are quite similar to the acoustic chambers wedges except they have ferrite dust embedded into the wedge material, making them incredibly expensive. We used them to get nearfield 3D polar plots of various types of microwave antenna prototypes.
On a dare from a coworker, I agreed to be shut inside for a while. These chambers have no electricity, so no lights. In the total darkness and silence I experienced auditory and visual hallucinations and a fairly intense subjective time dilation. After a bit I could hear my heartbeat, eyeblinks, a faint whooshing in time with my pulse, which I took to be blood running through my body, along with a phantom music soundtrack that I frequently perceive even in non-silent settings.
It was pretty eerie. And perhaps strongly influenced by a decade of my youth devoted to using various psychedelic drugs a few times a week, as the color patterns and music sounds I experienced were similar to what I experience in any dark silent place.
After my coworker opened the door, he told me it had been about 3 minutes, though subjectively it had seemed perhaps a half an hour.