Normally it should take 2 or 3 days,
The process is : 1st day you will have to submit application for extension, they will give you a yellow colour slip to come back and check next day or after 1 day,
you can know is your extension application is approved at that time on the second check only, if it is approved then collect that application with approval signature from that counter and take it with original passport to the visa extension counter, pay fees, and get the extension stamped (sticker) in passport.
The extension counters do not work on weekends and on weekdays only during the morning shift.