For as long as I can remember the Boonex Dolphin Technical Requirements Page (http://www.boonex.com/trac/dolphin/wiki/DolTech) has stated under the Server Requirements for Media Streaming section that you should allow at least 15 KB/sec for media streaming for each user.
Now that obviously means that it could use more than 15 KB/sec, but when a number is mentioned I tend to assume that it would be somewhere near it.
Recently I performed several RMS bandwidth tests. These were not extremely scientific, but it did allow me to get a better idea of how much I could expect, and whether or not the 15 KB/sec is an accurate number.
What I used for testing:
A default Dolphin install with the standard RMS and Flash App settings in the Administration panel.
I created 2 Dolphin test user accounts. One was broadcasting live audio and video by means of the chat feature. The other was viewing the audio/video. So, one broadcasting, and one viewing it.
I let this run for roughly 5 minutes. It wasn't exactly 5 minutes because it took a few seconds to turn on audio and video, and a few to turn it off when I was done, but it was close.
After the 5 minutes passed, my network monitor reported approximately 65 MB of bandwidth was used. Now if we split that between the two it would be 32.5 MB's. If we break it down even more it works out to be around 110 KB/second each. Much more than the 15 KB/sec quoted.
If you keep on going with the 1 person broadcasting a live audio/video feed in chat and one viewing for 1 hour non-stop that would be 65 MB x 12 = 780 MB per hour. And, 780 MB x 24 hours = 18720 MB per day (A little over 18 GB's per day). Lastly, 18720 MB per day x 30 days (1 month) = 561600 MB per 30 day month or around 560 GBs per month.
That would be for one person broadcasting both audio and video and one person viewing it non-stop 24 hours per day for an entire month in chat. This most likely would never happen, but 2 users could potentially generate 500+ GBs of bandwidth in a given month. Even if the same 2 members weren't using it all the time, as long as at least 1 member was broadcasting and one viewing at any given time it definitely would be possible.
Now imagine if you had a very active site with a lot of audio/video chat or IM going on. You could easily chew up a sizable amount of bandwidth.
There is no 100% accurate way to predict how much your site will use, but it can add up in a hurry if your members are using it on a regular basis.
I also tested it on a combination of CentOs and Ubuntu operating systems, with Apache, as well as Nginx. I didn't notice any substantial difference between them. Although the actual chat page loaded slightly faster with Ubuntu and Nginx, but almost unnoticeable. All of them had approximately a 1/2 to 1 second audio delay, which is more than acceptable. However, I reside in the U.S., and the test server is also in the U.S. So, an audio delay may be more apparent with a long distance connection for instance if the server was located in the U.S. and the member was located in Africa, Asia, or Australia.