How often can a relic’s Raise Ramparts gift be used?

Relics often allow the wearer to cast a spell, but do not specify how often. For example, the Raise Ramparts major gift of the Earth gifts allows the wearer to cast Wall of Stone. It does not specify a frequency, but if it has no limit it seems incredibly overpowered for a gift that can be handed out as early as 9th level. How often should this gift be able to be used? (I do not have the Gamemastery Guide, so maybe there’s just something that is missing or unclear on Archive of Nethys, but I cannot find any mention of a frequency limitation).