I've tried to look around for data concerning how much of a bandwidth hog a chat application is.
In this case maybe with a Java/AJAX implementation or simply just Java, using Server/Client relationship.
I want to find out, how much bandwidth such a system would use when it's written in Java. The benchmark could be 15-20 users from all over the world and peaking at maybe 8 or 10 max connected at a time. I know it might seem vague, but I simply can't seem to find data on this specific situation.
Can anyone point me to some resources regarding this? Or chip in if possible?
The reason you can't find data about this is because there's nothing particularly Java- or AJAX-related here. Bandwidth usage depends on the data you send/receive over the network, and therefore is dependent upon the protocol that you design to pass data around; it has nothing to do with whether you use Java only, or AJAX in combination of Java, or CGI scripts, PL/I or Assembler.
You can code a chat application in Assembler that will be a worse bandwidth hog than a chat application coded in Java.
In order to know your bandwidth impact, you need to analyze your data model, data flow and your overall communication protocol: namely, what data is being sent, in what structure, and how frequently.