well, that will depend on a few things:
is the session video, or audio, or both?
if video, is it a stationery or motion video?
how many people will speak at the same time?
just a few here... in any case, if u have a full motion video session, u are gonna use alot more bandwidth coz of the rate of frames per second.. typically, this would be about 128Kbps... for a fairly decent connection..
if u throw in how many people are gonna be talking on the connection at the same time.. u might use up more bandwidth so everyone's voice can bet transported to u... basically, the less voices u have on the session, the less bandwidth u'll use up....
u could use MRTG stats against your server/router during the session to get a rough idea of how much bandwidth u will use.. check out
good luck