Steam Wallet codes work just like gift cards which can be redeemed on your account for Steam Wallet credit and used for the purchase of games, software and any other item you can purchase on Steam.
 
Format: Code
Expiry date: Never expired
How to redeem: 
1. Go to http://www.steampowered.com/wallet and sign in if you aren't already. You will need to create a Steam account if you do not already have one.
2. Once logged into Steam, please enter your unique Wallet code that you received into the “Steam Wallet Code” field.
3. Click "Continue" to redeem the credit onto your account. Note: If you have never used Steam Wallet credit on your account, you may be prompted to enter your current address to determine the correct currency.
4. The value of the Steam Wallet code will be added to your account's Steam Wallet balance. The current available balance will be listed in the upper right hand corner below your profile name.
 
T&C:
1. The Steam wallet code is never expired
2. Each Steam User Account is associated with a specific currency, which you can see in the top right corner of your Steam Client. You may encounter an error if you try to activate a Steam Wallet Code issued in a different currency than the one of your Steam User Account. Steam TW is only for TWD currency account
3. for more details, pls refer to https://help.steampowered.com/en/