[Dovecot] Dovecot-imap, tls, gnus

I have a connection to my dovecot server from gnus:

,---- | (add-to-list 'gnus-secondary-select-methods | '(nnimap "hermes" | (nnimap-stream starttls) | (nnimap-address "hermes") | (nnimap-authinfo-file "~/.authinfo"))) `----

If I enter a group and read my mail then wander off and come back after a few minutes I can not navigate out of the group because my nnimap session has ended. What config parameter do I need to change in Dovecot to keep it open for the life of the client? (I am assuming its a dovecot issue here since it didn't happen with courier-imap).