When you enable two-factor authentication your withdrawal will be approved instantly, unless you have logged with a new IP address in the previous 24 hours.
A complex rule-set is used to determine if your withdrawal can be immediately processed or if there needs to be a manual approval process on our side. For example, enabling two-factor authentication you will satisfy some rules requires for the automatic processing of your withdrawal.
For more information about automatic withdrawal processing, log in and visit https://www.bitfinex.com/withdraw/greenline. Most automatically processed cryptocurrency withdrawals will be processed within 2 minutes, but processing can sometimes take longer if an approval is required or if our "hot wallet" (special wallet used to process withdrawals) is empty and needs to be refilled. The funds kept in our hot wallet are kept to a minimum as a security measure, and are refilled periodically as needed.