The official rule for the post office (for all of Spain I think) is that they will store it for 14 days before it is sent back to where it was sent from.
The post office in Santiago, since they are a pilgrim destination, say they will keep it for longer than 14 days, but they will not tell anyone for how long they will keep it. Many pilgrims recover their parcels from the post office after having walked for 30 days, and all is good. Once in a while you hear stories that something was sent back to where it was sent from before the 30 days, and that becomes a problem.
I offer this service:
https://www.caminodesantiago.me/luggage-storage-in-santiago-de-compostela/
Totally optional. It has a cost, but I will keep it for as long as you like (I say 60 days, but will keep it linger if you need more time).
Buen camino!
Ivar