16 C.F.R. § 309.16
You must keep for one year any delivery tickets, letters of certification, or other paper on which you based your posting of fuel ratings for non-liquid alternative vehicle fuels. You also must keep for one year records of any fuel rating determinations you made according to § 309.10 . If you rely for your posting on a permanent marking or permanent label attached to the electric vehicle fuel dispensing system by the manufacturer, you must not remove or deface the permanent marking or label. The required records, other than the permanent marking or label on the electric vehicle fuel dispensing system, may be kept at the retail outlet or at a reasonably close location. The records, including the permanent marking or label on each electric vehicle fuel dispensing system, must be available for inspection by Federal Trade Commission staff members or by persons authorized by FTC.
16 C.F.R. § 309.16