For a 2-inch trap arm, the maximum developed length from the trap weir to the vent (per the CPC trap-to-vent table) is approximately…

Explanation

The maximum trap-to-vent distance grows with pipe size: about 2 ft 6 in for 1-1/4 inch, 3 ft 6 in for 1-1/2 inch, and 5 feet for 2-inch pipe. Exceeding the distance risks self-siphoning the trap because the vent is too far to relieve pressure. The trap arm must also maintain proper slope within this length.