When creating your DR plan, you should be looking at every aspect of your business and determining how long you can be without a specific function.
Then you try to determine the impact on business, customer relations, etc for every N period over that amount.
For instance some business may be able to have an FTP server be down for one day without a great impact and anouther business it might be one or two hours.
The same for internet access, and other processes.
Once that is done, you deterime the ROI on setting up your redundacy systems and you should have all the specs ready so that you are not making the tought decision at the time of the disaster. Rather you are following the plan.
A disaster exists when critical business processes are sufficiently impacted such that down-time procedures cannot prevent unsustainable harm to the organization. For example, down time procedures can typically keep critical processes going but at some point, recovering from the use of those downtime procedures (re-entering all the data accumulated during the downtime) is impossible.