Hi Phil,
Thanks for posting back.
When you apply a retention policy to OneDrive account, a Preservation Hold library will be created.
If a person attempts to change or delete content in a site that's subject to a retention policy, first the policy checks whether the content's been changed since the policy was applied. If this is the first change since the policy
was applied, the retention policy copies the content to the Preservation Hold library, and then allows the person to change or delete the original content. There, a timer job runs periodically and identifies items whose retention period has expired, and those
items are moved to the second-stage Recycle Bin, where they're permanently deleted at the end of 93 days.
So if you have a 10 years retention policy, the 10 years is applied to retention container(Preservation Hold library), and the recycle bin and the second-stage recycle bin is 93 days.
I suggest you check the information in "How a retention policy works
with content in place" from the article I shared.
Best regards,
Tina