The exact method for map caching has not yet, to my understanding been clearly defined, no-one is sure if there is a particular amount of recently accessed mapping space reserved for caching, or if it works in a way similar to other apps and if the systems demands free space it could come along at any time and remove the temporary files in the cache.
There may be a formal method for achieving this in the new iOS7, or it may stay the same, i.e. an unknown function.
Anecdotally I can confirm that caching does work, for relatively large areas of mapping, having left the UK on a trip to DisneyLand Paris without data access - I previously "travelled" the route I intended to take at a reasonable zoom level from the point of entering France to the destination, (~5 hours driving) and was able to use GPS tracking overlaid on the cached maps for the entire journey there, and back again 3 days later. Whether it was necessary to pre-travel the route, and whether the zoom level affects anything I can't be sure (I suspect the maps details, as vectors, didn't care, but naming points and address overlaps may have required it for more detail).