How long does a Genius Bar display repair take?

Unfortunately there no way to answe that specifically as it will depend on the extent of the issue. If its simple it may be done while you wait.

If it's too complex, and you have Apple Care+ they can just replace your Mac with a different service unit on the spot.

Otherwise any repair time frames will be provided when they have diagnosed your Mac and know exaclty what is wrong with it and what may need to be done to repair it.

Also note, time will vary if they have any required parts in stock or must order them in for the repair.

[In my opinion] the fastest typical in-store repair takes three days elapsed time provided parts are already available in the store, and technician-time is readily available. If parts need to be ordered, it will take longer.

[In my opinion] the fastest typical depot repair takes ten days elapsed time, because it includes two weekends and transit time, as well as parts delays and technician time availability. If parts need to be ordered, it will take longer.

[In my opinion] if it gets done sooner, Merry Christmas! -- you get an unexpected present.