RT60 is a standard measurement used to denote the time it takes for a reverb sound to decay by 60dB. In general here is common reference for RT60 times:
- Generic room = 0.8 secs (800ms)
- Generic Medium Hall = 1 sec (1000ms)
- Generic Large Hall = 2.5 secs (2500ms)
For example, if you were to clap your hands in a generic large hall and it takes 2.5 secs for the early reflections to decay to a level of -60dB then obviously the RT60 will have a time of 2.5 secs.