Anyone know if Myles the NPC who joined me in Anika stay with the party? Or does he take off after a certain while?

He does take off. None of the RPCs come with you absolutely everywhere, though a certain one of them does come almost everywhere.

Thanks for the response ;)

Just to clarify: the limit is solely by location. The amount of time passed doesn't matter.

You'll need to juggle NPCs at least a couple of times as you play (whether that means switching to different ones or just leaving someone behind for a while).