Depending on how tough the bolts are to remove and how hard it is to get the brake calipers and brake pads apart it should take around 1 to 2 hours at most to replace the brake pads.
I replace my own brake pads and sometimes it takes me an hour for both sides and sometimes it takes me 2 hours depending on how hard they're to remove.
Some bolts get stuck and others are easy to remove so it's can vary.
A good shop should be able to replace the brake pads within 1 to 2 hours.