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).
The gift referenced is at will
Barring GM intervention, there is no limit on using that Relic ability. Contrast this to Living Statue
You can cast 6th-level flesh to stone and stone to flesh as innate spells, each once per day.
If a gift has restrictions on number of uses, they are included.
That being said, a GM who is worried that overuse of an ability will adversely affect their game (eg they have a sneaking suspicion that cities will suddenly have walls built around them overnight and be held for ransom) gifts can be altered to add a limit per day or else simply not award that gift. Per the general rules on Relics, it is assumed the GM is making the decisions about which gifts to provide. Alternatively, many GM's still use the "actions have consequences" method and will harass characters who abuse their Relics with all forms of misfortune, from bounties on "Waaaaally the Wall Maker", angry earth elementals, etc.