Pre-caching limitations and reliability in Google Maps
The number of areas you can pre-cache is limited by the amount of system storage you have available.  It is important to note that the cache is stored in system storage, not the SD card.  Also, the stored data is the base map tiles and landmarks.  You will not be able to get directions based on this downloaded data, but you can browse it offline and track your current location.

The cached data expires and is deleted automatically after 30 days.  If you are not able to refresh your offline data in that time, you may be in trouble.

More data on the feature is available on the Official Google Blog post.