I tell the client that I keep their files for one year. Before I delete them, I check with the client to make sure everything is safe and there are multiple copies in different storage locations including cloud storage.
No client has ever come back to me regarding metadata mistakes. I use facial recognition to identify and name people, and I review that with the client either in-person or virtually – so they can’t blame me for assigning the wrong name to someone.
As for mistakes with keywords or dates, I have a system of gathering info from clients, and mistakes have never been an issue.
If I did truly make a mistake, I’d fix it for free. If a client wanted metadata added or changed, I’d charge an hourly rate. If the client was unwilling and/or unable to add or change metadata – well, that why they hired me in the first place, and they’d likely be willing to pay for additional work.
If they do want to add metadata themselves, there is consumer-friendly photo software for PC. I’m still looking for a good, relatively easy Mac solution other than Apple Photos.