I'm not a 100% sure if the sessions time out or not.
I checked the logs on my pfsense firewall which handles the pppoe login/handshake and the last entry is from the 22nd and then before that the 14th. Both those dates correspond to firewall reboots. So I don't think they expire or if they do it's a long time period before they do.
Please correct me if I'm wrong.