Best practices for hosted tiled map services
Tiled map services run quickly on the web, but it takes an investment of server power, time, and storage space to build and maintain the tile cache. When you host your services on ArcGIS Online, acquiring server power is not as big of a concern as it would be if you were creating the tiles on premises. Esri maintains the server and is responsible for completing your tile creation. However, you need to prepare for the time that it takes to build a cache. You also need to anticipate the amount of disk space the cache will use.
Viewing cache status
Anytime after you start creating tiles, you can display a scale-by-scale progress report of the percentage of cache your server has generated. Right-click your tiled map service in the Catalog tree and click View Cache Status. You'll immediately notice that the larger cache scales take much longer to cache than the smaller scales.
You can also see the cache status when viewing the item details of the tiled map service in your My Content page in ArcGIS Online.
Staying within your limits and building tiles strategically
Your organization has purchased a finite number of credits that can be exchanged for ArcGIS Online services such as building and storing tiles. The number of credits remaining for your account is visible to administrators in the My Organization page under the Subscription Status menu. If you exhaust all your credits while creating tiles, the server stops the tile creation and you will need to acquire more credits or use only the tiles that already have been built.
If you're responsible for caching a broad area at large scales, you can save credits by only creating the tiles that are most likely to be viewed. In some rural areas, oceans, and so forth, there might be little chance that a user would zoom in to the larger scale. The caching tools allow you to define an area of interest to constrain tile creation. You can define an area where you expect tiles to most likely be viewed, thereby prioritizing your tile creation and saving disk space.
It may take several iterations of the Manage Map Server Cache Tiles tool to build the cache the way you want. Your first run of the tool might cover the small and medium scales over the full extent of your map. The second run of the tool might cover the large scales over just the areas that you expect to be frequently visited. The extra work and planning required to manually run several iterations of the tool may actually save you some time and money if you can reduce the number of unneeded tiles built.