In certain scenarios, they are persistent
For example if you chase an NPC in normal space and they high wake out, you can scan the wake, jump in that system, and you will find them in Supercruise and they can be interdicted.
I used to pirate some npc like that. Since it was a fringe system, they jumped in anarchy/unpopulated systems and the resulting cargo was not marked as stolen
However, it's been a while since i did that - not sure if it was changed.
Edit, but to answer to OP, nope NPCs do not refresh or deplete the stocks at stations.
These depend on system parameters and further affected by BGS states.