Reverb decay time controls the duration of reflections in a space to run out of energy by fading out to a level were they become inaudible.
In most cases the decay time is configured to lose energy from 1khz at a time governed by the RT60 measurement.
Basically RT60 refers to the time it takes for a burst of sound in a room to attenuate to a level of 60dB.
For example, if you produce a sound in a room and it takes 10 seconds to fade out completely then that particular room has an RT60 of 10 seconds.
How To Set Reverb Decay Time
The general convention is that a large room is suppose to have a longer decay time and a small room should have a shorter decay time.
The reason larger rooms have a longer decay time is because the distance between the surfaces of the space is bigger than say in a small room.
In nature it sounds wrong to have a large room with a short decay time. Trained ears can pick that up as compared to novice listeners.
So if you want your reverb to sound realistic and not weird then stick to those general guidelines of setting the decay time.
As for experimental purposes you are free to break the rules and do the opposite by having a large room with a very short decay time.
Reverb Decay Time Tips and Hints
If you are working with material which sounds busy and uptempo then consider using a shorter decay time and a small room or space.
Shorter decay times are associated with tightness and allows instruments to have their own ambiance without masking the clarity of other elements in the mix.
Longer decay times are useful in cases were a song isn’t busy otherwise there will be intelligibility issues and will result in a washed out mix especially for vocals.
When working with percussive instruments then it makes sense to set the decay time with the tempo of the music. This will create a coherent rhythmical effect.