An approximate cooking time of 3 hours is recommended for a medium-sized rack of ribs. If the ribs are too tiny, or if you are unsure about your barbecue setup, start inspecting them after around 2 hours of cooking. A thick slab may take up to 4 hours to complete. If you’re cooking more than one slab on a vertical rack, allow an additional 30-60 minutes for each additional slab.