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tinyurl.com/bt5o4ceTrash Folder
Yahoo! Mail deletes the messages in your Trash folder at occasional intervals, and it is not possible to control the time of deletion. Don't expect emails to remain in Trash for any amount of time.
Best bet is go by the basis of once a month. It seems like it's whenever Yahoo! deems it is necessary to clean up the servers. Either way, you will want to make sure you don't store stuff that you may remotely need in the trash, especially if the Frontier e-mail account is a Yahoo! Plus account with unmetered mail storage.