These figures will depend greatly on what you are going to be using JBoss for.
If you deploy a tiny app with only a couple of users you can configure JBoss to use less resources.
If you deploy a huge application with lots of users JBoss would need a lot of resources.
As JBoss is a Java application, you basically configure the maximum amount of memory it can use. Of course if you don't give it enough memory, it will crawl.