I don't think any long-term implicit consent is acceptable. I would not expect that after opening one document in a folder without being shown any permission prompt, that permissions have been permanently altered. I would never even go look to see if it was "implicitly permitted".
Without a prompt or notice, I would expect only that the app has access to the file or directory I chose until the app is closed/quit.
Why should the permission even persist that long? You might leave that app running for the next two years.
Shouldn't a temporary access be temporary? Possibly scoped by time? Possibly scoped to a single access?
Because the app may generate more than one descriptor for it or perform more than one read or write operation in the normal course of usage. If I open a document, and come back to it 6 hours later and click the save button, I would expect it to save the document.
How would the app be able to reopen the file then?
It would ask for permission.
Every time you relaunch the app?
It depends on the app whether that would make sense. If it is document centric, then yes. The user should explicitly open every time. If it doesn't make sense for the user to open it every time, it should ask for permanent permission and that should be recorded in system settings where it can be removed.