When you create map views using the online map provider, Tableau stores the images that make up the map in a cache. That way, as you continue your analysis you don’t have to wait for the maps to be retrieved. In addition, by storing the maps, you can do a certain amount of work when you are offline. For more information, see Select Background Maps(Link opens in a new window).
The cache for the maps are stored with your temporary internet files and can be cleared at any time by deleting the temporary files from your browser.
Stored map images and legends remain valid for about thirty days. After that time, Tableau will not use the stored image; instead, it will require you to reconnect and fetch an updated map. This is to prevent the map images from becoming outdated.