With the box being a 20mi x 20mi area (400sq/mi), I would think that you normally wouldn't need a map the entire way. Just a cached map of your destination where you're no longer traveling several miles on the same highway or county road.
And too, with the ability to save 10 maps, that's 4000sq/miles of map you can utilize. That would easily accommodate a cross country trip across the US if you just cached the areas of turn off points and your final destination.