49 C.F.R. § 192.721

(a) The frequency of patrolling mains must be determined by the severity of the conditions which could cause failure or leakage, and the consequent hazards to public safety.

(b) Mains in places or on structures where anticipated physical movement or external loading could cause failure or leakage must be patrolled—

(1) In business districts, at intervals not exceeding 4 1/2 months, but at least four times each calendar year; and

(2) Outside business districts, at intervals not exceeding 7 1/2 months, but at least twice each calendar year.