In Web Mail preferences you can set the automatic deletion of the Trash Folder to 1, 7, 14, or 30 days, or Never. Currently there is a bug where Trash is auto-deleted at a maximum of 14 days, and settings higher than that are not being honored.
This bug also exists for the Suspect Email folder if you are using spamBlocker on High. That folder has the same settings.
UPDATE - This bug is now expected to be fixed in around the end of March, so that setting 30 days or Never will work correctly. In the meantime, please don't put email in the Trash that you want to keep permanently.