Withdrawals are usually broadcasted on the blockchain network immediately after initiating any withdrawals, but in some cases, they are delayed due to network load.
If you decided not to go along with the withdrawal, you can cancel if the transaction is yet to be broadcasted (i.e., it has not been assigned a transaction hash) by going to the Account page, under the “latest transactions” tab or on the payment history page and clicking on the red cancel icon on the right from the withdrawal status.
If the option is not available, it means the withdrawal has been broadcasted on the blockchain network, and waiting to get the required network confirmation.