The time it takes to repair a transmission depends on what is actually wrong with the transmission and what is getting or needs repaired on the transmission.
If it's just an transmission filter that needs replaced or some other solenoid or part in the transmission then it might only take a few hours to repair the transmission.
Now if it's something else major or the transmission needs rebuilt then it could take a few days to a week or more depending on how busy the shop is and how hard it is to remove your transmission.