World-of-warcraft – What happens to unsold items on World of Warcraft Auction House

If I remember correctly, the deposit gets returned to you when something sells, but the auction house itself takes 5% out of the winning bid as it's fee. I do know that it's a fixed portion, and not affected by duration.

The Neutral Auction house takes a larger cut -- 15%. Filthy goblins.

As a reminder, you don't get your deposit back if your item doesn't sell. The cost of the deposit itself varies with duration (as you noticed) but the vendor price of the item in question as well.

Taken from WowWiki

Faction-Based Auction Houses:
Deposit (12hr) = 15% of Merchant Sell Value
Deposit (24hr) = 30% of Merchant Sell Value
Deposit (48hr) = 60% of Merchant Sell Value

Again, the goblin-run Auction Houses charge more:

Neutral Auction Houses:
Deposit (12hr) =  75% of Merchant Sell Value
Deposit (24hr) = 150% of Merchant Sell Value
Deposit (48hr) = 300% of Merchant Sell Value

I have a terrible habit of not checking my mail before freezing my account, and I can confirm the clock stops for no man: after 30 days, the mail is returned, and after 30 more days, it's deleted.

Besides the clock, the mailbox works as if you were still subscribed. Your characters, for all intents and purposes, are considered merely "logged off" until you explicitly delete them, so mail will still be delivered even while your account is frozen.