The rescan will take a while.  The exact amount of time will depend on the processing speed and disk access speed on your computer.

Once the rescan is complete, the bitcoins will immediately be added back to your balance.  It would be a really good idea to create a backup of your wallet before you start this if you haven't done so recently (just in case anything unexpected happens).

 

All transactions will be removed from your wallet if you restart it from the command line with the -zapwallettxes command line option.  It will then rescan the the blockchain to find all confirmed transactions and add them back into the wallet.  Once those unconfirmed transactions are removed from the wallet, and the blockchain has been re-scanned, the wallet will show that 0.12 BTC value as being available for spending again.

This doesn't stop those transactions from possibly eventually being confirmed and removed from your wallet again though.  To do that, you'd need to get a new different transaction confirmed that uses the same input as the 0.08 BTC transaction.

As RodeoX has pointed out, the problem is that the 0.08 BTC transaction didn't pay a transaction and the 0.04 BTC transaction is dependent on the other transaction confirming first.

It might be best to just remove the two transactions from your wallet and re-send them using coin-control features of the wallet and paying a reasonable transaction fee.

To make it easier to help you, please supply as many of the following as you are comfortable sharing:

• bitcoin address that you sent to
• transaction ID's of each of the transactions
• Which software wallet or online service you are using to send the transactions
• If you are using a software wallet, then which version of that software you are using