The default timeout the JBossMQ Server uses before it assumes a client is dead
is 2 minutes. See the read timeout setting in uil2-service.xml
When a client dies, JBossMQ will nack all the unacknowledged messages.
As I have said so many times in these forums....
Throwing an Exception from a MessageListener is not allowed by the JMS spec.
On JBoss it will result in the message being dropped (positive acknowledgement).
For a demonstration of the client acknowledge behaviour, see the example on the WIKI.