In a room made entirely of mirrors, the reflective surfaces would indeed bounce the light around, causing multiple reflections. However, the brightness of the residual light would diminish over time due to the nature of light absorption and the limitations of perfect reflectivity.
When light reflects off a surface, a small amount of energy is typically absorbed by the material. Even in the case of high-quality mirrors, there is still some absorption that occurs with each reflection. This means that with each bounce, the light loses a portion of its energy.
As the light continues to reflect off the mirrors, it undergoes multiple reflections, and each reflection results in a further decrease in intensity. The absorbed energy is converted into heat, further diminishing the brightness of the residual light.
Additionally, no mirror is perfectly reflective, and there are always imperfections present. These imperfections cause scattering and diffusion of light, leading to an overall decrease in the intensity of the reflected light.
In practical terms, the room would gradually become darker as the residual light is absorbed and diffused with each reflection. Eventually, the remaining light would become too faint to be noticeable to the human eye.