Welcome to the forums,
See the individual or COS > advanced tab.
Currently you can set:
Email message lifetime-Number of days a message can remain in any folder before it is automatically purged. The default is 0; email messages are not deleted.
Trashed message lifetime-Number of days a message remains in the Trash folder before it is automatically purged. The default is 30 days.
Spam message lifetime-Number of days a message can remain in the Junk folder before it is automatically purged. The default is 30 days.
The ability to give some control to the user about their trash/junk(spam)/chats/sent/inbox lifetimes is being built into v5, but will not be out UI side in the initial release. See:
/forums/administrators/11243-zimbra-5-0-beta-3-user-definable-expiration-policy.html