When should we use an external content management server for document storage?

We have a growing list of use cases for attaching documents (documents, pictures, PDF's) to objects via document management and "services for objects". Currently, we do not have an external content repository, so the documents are stored in the SAP Database. We are concerned about the scalability of this approach.

Are there any guidelines for document storage requirement (number of documents, storage space required, etc) that indicate when an external content server is the appropriate solution (in lieu of the SAP Database)?