Recessed lights take on average 30 minutes to 1-hour each depending on how difficult it is to fish the wires in the ceiling. With this, you also have to expect time for protecting the area first with drop cloths, measuring out the room, laying out the lights, cutting all the holes to fishing the wires, installing the recessed lights and the cleanup process when it's all done. Installing a group of 4 lights on average would take about 4 hours whereas installing a group of 6 lights may only take 5 hours.