Analytics Index Training Data Source Rule of Thumb
Note that the RelativityOne Analytics Guide (available here) provides a guideline for when it will be necessary to add new documents to a training set. With as many as 5,000 new documents it may not be necessary to update the training data source, if you have a total of more than one million documents in the data source. However, if 100,000 documents are added, the training data source set should be updated for the analytics index since the likelihood that the new addition will contain new concepts increases.
This is a useful rule of thumb, but the key criterion is whether or not the newly added documents differ significantly from the data source for which the analytics index was already created.