As early as possible.
They say "aviate, navigate, communicate", which puts navigation into a pretty high priority - just behind flying the plane. It is more important than talking on radios.
I cannot see any downside for learning navigation too early beside the natural "memory decay" for acquiring any new material & knowledge. Even basic navigation techniques will benefit your first day of flying (unless you're absolutely not interested in knowing where you are). You can even discuss the route with your instructor during the pre-flight, even if flight planning is not the lesson today. You will not get to know the details, but you will be introduced to aerial navigation.
That, of course, assume that you are learning at a comfortable pace and you are not struggling with the present material.