Data Retention best practices

I do not know of any best practices. In the end, you will have to store the date when to handle deletion for each document or record. Then schedule a cleanup process that manages the required tasks.

This is as close as you'll get: docs.appian.com/.../Archiving_Processes.html

Data retention rules are usually primarily dictated by the organisation itself and sometimes by technical / costing considerations (i.e. price of disk space).

For documents specifically, I would generally advise on moving them to a CMS at some point in their lifecycle.