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
They should still be in the Trash folder if you only deleted them a few days ago. If not, right-click on the Trash folder and you will see the option to "Recover Deleted Items". Click on that and see if they are in there. But for longer term storage, you should create a folder and put messages in there that you might want to look at later.