In the free version of the app, when do automatic virus definitions updates already occur?
Today November 20 is the update to November 19.
Can you implement time intervals for checking / updating definitions?
This is from the Avast website,
If your system is on-line, Avast receives a streaming update approximately every 6 minutes
and provides 2 major protection updates per day.
Remember, there are always exceptions.
Yet mine is always connected and does not update immediately … :-
Sometimes it lags 1 day with the definitions, I have to update it manually.
It would be useful if they put an update time interval that the user can choose. 
If I remember correctly in previous versions this was the case.