Mine is set to 240 the default as in the image (there really isn't any need to change that), you only get notified if there is an update available. Dial-up has so many issues, poor connection speed, dropped connection, etc. and these are most likely to happen when you have just connected that it doesn't warrant setting it to automatic. Now if this happens when avast is checking for an update and you lose the connection you will get an error in the avast update routine, this throws many people as they think it is a problem with avast, rather than their connection.
Now you have the line, leave it and change it back to the default 240 using the same interface you used to change it to 30.
The VPS updates (or iAVS Update using manual update function) occur about 3-4 times per week, however there have been on occasion 3-4 in a day depending on serious virus activity on the internet.
Further to your original post:
Anyway, I never had an automatic update. It might be, I stay connected just for 5-10 Minutes for each session.
How long was it that you waited, days, weeks, that you didn't get an auto update (bearing in mind the frequency I mentioned, there might not have been one. Because your connections are so short I would suggest that you switch to 'Ask when an update is available' that way you know one is available and don't disconnect prematurely.