Why is there absolutely nothing cached by default? How many times does the topology of the Atlantic Ocean change every day that an outline cannot even be shown? I get not caching high zoom levels, but there is literally zero optimisation here done.
That said, if you don't like what Google Maps does, there's lots of other choices. Here maps (from the Navteq/Nokia lineage) had pretty reasonable downloading options when I was using it on WP and is available on platforms that lived.
I think I would go for 1 month cache. Balance the possibility of change with the likelyhood of no change.