In a recording studio the reverberation time is the average time it takes for all frequencies to stop after a burst of white noise. Usually a control room would have a reverberation time of between 0.5 and 0.8 seconds.
You can use free spectrum analyser software attached to your computer with any old microphone and speaker to determine your reverberation time. The important thing to note is that all frequencies reduce at the same level, you don’t want the low-end to last longer than the high-end, as that will make your mix bass light as the repercussive effect of the low frequencies taking longer to die down will cause you to think the bass is to heavy and subsequently mean that you will reduce the amount of bass. Only when you listen to the track outside the environment will you see that the track is lacking bass.
It works the same way for high frequencies, if the high frequencies have a shorter decay (common in project studios, because of the lack of knowledge with acoustics) the end result would be a top heavy sound track.
So, as you can see, it is important to make sure all the frequencies reduce at the same amount.