As is being seen in the forums this is a very haphazard issue, for a program that supposedly no longer requires registration, it sure looks like it does.
Hoverer, this is a bridge you don't have to cross at any time soon. You would get notifications before that time comes, exactly when I don't know as I have never got a notification. As I have said even if you do get them and the expiry date does come, avast free will still continue its protection and will eventually update the registration.