After authorization with scope : offline_access. It is possible via application to post to wall, or perform any other allowed operations at any time, even if user was logged out some days ago, using the access_token? I'm asking, because I couldn't post to wall after later, facebook does not allow that.
Facebook will return the code value and with the help of of that you can get the access_token which you must have to store in database for future use. That access token has no expiry date so you can use that any time in future.
It can be possible that user might have remove permission to posting on his wall after granting the access from your application. but it will going to ask that user again when he's going to access the app next time.
Thanks!