If you are talking about saving them for audit reasons, we keep ours for 5 years. Our company is in the FI category.
If you are talking about just as a library, we keep ours as long as the system or product exists but we have to be on top of updating them as things change.
I don't think there are any industry legal requirements to maintain a manual test case. There are couple of things that you should be aware of:
1. If you have test cases maintained in a tool that saves history, then great. You should be ok as this will help you see revision history of test cases.
2. Maintaining manual test cases for long should consider better automating them.
3. If your software is outdated or is expired and manual test case is no more valid, then it's on your discreation to keep the software documents for next 5 years and then later destoy it.
4. It really depends on situation and vision for the software those manual test cases are being maintained for.
StickyMinds is a TechWell community.
Through conferences, training, consulting, and online resources, TechWell helps you develop and deliver great software every day.