How much of Google Maps is stored in memory on my mobile device?

How much of Google Maps is stored in memory on my mobile device?

It obviously has to be the case that the map is stored in memory.
However, when I calculate my route then turn off my wifi/3g, Google Maps
still navigates me successfully. It is also the case that, when I
pre-calculate the route, lose connection and then make a wrong turn, I am
still successfully redirected given that I don't stray TOO far from the
given path.
So how does Google Maps determine HOW MUCH of the route to store in
memory? And how is the data structured, some sort of weighted graph?