A common problem is a user checks out a document, makes no changes to the document, closes the document, but fails to check back in the document. I know Nintex has an option to check back in the document, but the workflow will only work if the document is new or has been modified. What if the document has been opened, then closed, but not modified ? How would I leverage a Nintex Check In object workflow to check the document back in ?
technically you could check in a document after a period of time but you'll never be sure that it was what the user wanted to. Before trying to find a technical solution for this I'd suggest you speak with the people involved in the process to check in the documents after they worked with them.
Additionally, document check may even be turned off to allow co-authoring.