It all depends on the type of sound. For the intellegibility of speech, the size of the venue is quite different to that for music:


The reverberation time (RT60) is proportional to the volume of the venue, that is, your listening room:


So in your case with a total volume of 2,587.5 cu ft, 300ms is about right. But these figures depend strongly on the absorption coeficients of the wall, floor and ceiling of the room, as well as those of the room's contents; and the coefficients are often frequency dependent.

Have you ever wondered why in cinemas, theatres and concert halls the seats are usually cushioned? It's partly for the comfort of the sitter, but it is also to mimic the sound absorption of a human body when the seat is unoccupied.

It's all a bit of a 'mine field' - so don't go overboard with it all!