There is no single number. The build gets matched to the traffic the floor will actually take, which is why a showroom and a warehouse get quoted differently even though both are called commercial epoxy.
Light retail sits at the thin end, a film system around 20 to 30 mils. Warehouses and service areas move up from there into thicker broadcast builds, because the floor is absorbing impact and abrasion rather than foot traffic. That extra body is what stops a dropped tool or a steel wheel from going straight through to the slab.
Then you spec by zone, not by building. Forklift lanes and loading docks take extra topcoat where the equipment turns, since that is where the tire twists in place and grinds. The open floor twenty feet away does not need the same build, and paying for it there is money that buys you nothing.
Working out what your floor actually needs? See how we size commercial epoxy flooring systems for St. Louis facilities to the traffic in each area.