Is there a certain number of years that BSA-related training material (proof of training employees and the material used) is to be kept?
We have material from 2017 and it is too cumbersome to scan; all since then have been scanned, thank goodness.
I am thinking that it only needs to be kept since the last federal BSA exam, or is it 5 years like other BSA retention?

No, there isn't a certain number of years that BSA training material must be retained, but some choose 5 years to go with the typical 5 year record retention requirement - but that isn't a hard requirement. You will want to maintain your records long enough for your examiners and auditors to review them as the BSA Exam Manual says this:

"Banks should document their training programs. Training and testing materials (if training-related testing is used by the bank), and the dates of training sessions should be maintained by the bank. Additionally, training materials and records should be available for auditor or examiner review. The bank should maintain documentation of attendance records and any failures of personnel to take the required training in a timely manner, as well as any corrective actions taken to address such failures."

Part of me sees this retention on two sides. 1) Proof to examiners it was done and 2) in the event an employee is fired for violating the banks policy/procedures it is more difficult for them to claim they were never trained on such issues. Not a likely issue, but still a benefit of retention.

Our BSA records are five years like the rest of our BSA records.