active mq (size of the internal data structure and expiration of messages)

[Suchitha Koneru \(sukoneru\)]

Hello Active Mq Users , 
                      I am using Publish/subscribe model for transfer of
messages with durable subscriptions. I suppose that active mq
internally stores messages using queue or a similar data structure.
 
I was just wondering about the size of this queue maintained internally
by Active MQ. Does it  overflow ? How will we know, what it's capacity
is ? 
Any insight in regard to this ?
Also , the messages place in this internal queue  will be  discarded  ,
once they expire ?  What is the time frame , within which they have to
be consumed ? is there a way to determine this?
 
thanks,
Suchitha