Best practices for offsite storage
There is not a lot of best practices information available about distance requirements for offsite storage, because those requirements vary based on the region and the specific threats associated with a particular geography.
There are obvious bad choices such as keeping backup media on a different floor of the same building or even in a neighboring building. That said, it might be acceptable in a particular area to have backup tapes stored in a vault a few city blocks away from the data center. In other areas, such as hurricane-prone regions, the tapes must be kept at a distance greater than the destructive reach of the weather phenomenon.
The bottom line is that backup tapes should be kept at a safe distance from the disruptive or destructive elements that may cause you to need those tapes. Best practices would dictate that a risk assessment be conducted to identify the threats, probabilities and impact. This information can then be used to establish the appropriate distance between production and backup data.