If you are using the blockchain.info wallet, they will wait a few days for the transaction to confirm. If it has not confirmed yet after a few days have passed, they will stop broadcasting the transaction to the rest of the network, and will show you your balance as if you had never sent the transaction in the first place. If you then send a new transaction that re-uses the same inputs, the address will be labeled in their block explorer web pages as having been associated with a "double spend".
if the transaction is not confirmed by ~2 days then they stop broadcasting it
for a short time i have not take the exact time it takes,
but it is not more that 15 minutes. EDIT: i checked my email, and apparently it is 1 hour gap.
then the same transaction will be re-broadcasted again.
the balance is restored in that
15 minute gap and you can double spend it.
also you will receive an email indicating that the transaction is failed.
Transaction Confirmation Failed
A transaction made through Blockchain.info has been removed from our database because it was taking a long time to be included in a block.
Any funds have been returned to your wallet and you can now spend the coins again. Please consider including a larger fee in future transactions.
after this it will be re-broadcasted to the network