see bottom of this page:

[u]http://forum.avast.com/index.php?board=2;action=display;threadid=1647;start=0[/u]

Just done this myself, I set UpdatePeriod to 60mins (in avast4.ini):

[InetWD] ;leave this alone
UpdatePeriod=60 ;<<<<< add here
UseRAS=1 ;leave this alone
AssumeAlwaysConnected=0 ;leave this alone

may need to reboot to take effect?