Documentation of a Business Case for IT Development Project should be retained until :

A business case can and should be used throughout the life cycle of the product. It serves as an anchor for new (management) personnel, helps to maintain focus and provides valuable information on estimates vs. actuals.
 Questions like, “why do we do that,”“what was the original intent” and “how did we perform against the plan” can be answered, and lessons for developing future business cases can be learned.
During the development phase of a project one should always validate the business case, as it is a good management instrument. After finishing a project and entering production, the business case and all the completed research are valuable sources of information that should be kept for further reference.