The PPPoE tunnel will always stay established. You cannot select "interesting traffic" to bring the tunnel up. It is always up on the PIX.
For the connections, UDP connections are torndown after the Idle timeout has expired, TCP connections are torn down after the FIN sequence is completed, or when a Reset packet is seen, or when the idle timeout expires. Once no connections exist on a translation, then the translation will be torn down after the idle timeout expires.
hope this helps.
David.