Use a backup facility at least 5 miles away.
For very small localized businesses, if the owner survives, a set of backups that can be recovered within days to weeks and the ability to purchase new hardware is adequate. Redundant data centers are not a requirement at all. To the extent that there is redundancy, most protection is against single facility failures such as the main place of business burning down or some service outage. Redundancy may be limited to a backup tape taken home with the owner at night or every week.
Use a backup facility in another city.
For strictly regional companies, city to city redundancy is called for unless the enterprise is single facility or very limited in geographical scope. By spreading across cities, local government failures, telecommunications outages, common energy and infrastructure failures, most riots and fires, many tornadoes, hurricanes, earth movements, most floods, and many other similar events can be survived, Care must be taken to assure that commonalities are avoided in each of the interdependent areas so that business continuity is assured. For example, placing all data centers along coast lines could be subject to common mode failures associated with increases in sea level. Cost differences are typically very small for assuring proper infrastructure redundancy, but expertise is needed to verify this redundancy.
Use a backup facility at least 250 miles away.
Many enterprises operate within only one continent or country, or substantially do so in terms of their need for information technologies associated with business continuity. In these cases, distance becomes an issue when regional events take place. In the US, for example, there are hurricanes that effect one region, earthquakes in another region, floods and storms in another region, winter related infrastructure failures in another region, and so forth. If the enterprise exists only within a certain region, then it doesn't particularly help to have redundant data centers elsewhere because they will be of little use when the rest of the business does not operate. The exception is the large enterprise that is dominant in a region but has enough diversity in operations to continue even when that region is hit by a regional event. Enterprises that cross regions usually have substantial facilities in more than one region, and these are often, but not always, ideal locations for redundant data centers.
Use a backup facility on another continent.
Distance requirements for backup facilities are driven entirely by scenarios that can lead to simultaneous loss. These in turn are threat driven. If an enterprise has to be able to survive limited nuclear attack, government insurrections, regional and world wars, and comet strikes that don't end all human life, transcontinental diversity is a necessity. There are many global multinationals that have this requirement and the efficiency gained by reducing this sort of redundancy does not justify the collapse of the business under conditions that, in the case of wars and governmental changes, happen quite frequently.