Are you still in the old email format or the new one? To tell, look at the URL in the address bar. If it says Zimbra you are in the old one, if it says Appsuite you are in the new one. If you are in the old one, on the Inbox page go to Preferences>>>>Email.>>>>Email Deletion Schedule. There you can set Trash for up to 30 days max. It's time-driven, and not any set number of messages. In the new format----------
Spam is stored for 7 days or 2,000 messages, whichever comes first
Trash is stored for 14 days or 5,000 messages, whichever comes first
You cannot change those settings.