How often does the mailbox refresh?

[Ruvanal]

Items take 1 hour.

Money and messages with no items are instant.
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Note that while the items will take 1 hour when they are sent; if you press the return button at the bottom of the recieving mail frame, the item will be returned to the sender immediately. This will not happen if you take the mail out and then remail it (incurring the 1 hour delay again).

Also the Auction House mailing will happen immediately when the bid is won.