avast just ping their websites to see if you're on line and to donwload updates. I did not check one by one these IP, but the list of IP pinged are on the server.def file (into
setup folder under avast installation folder).
avast checks if there is an available connection each 40 seconds. If there isn't, wait more 40 seconds to check. Checking does not take more than one second and, of course, does not use the Internet band. If there is a connection, check for an update. If there is not any new file to download, wait 4 hours to start checking again. If there is an available update, start it and install it. Again, wait 4 hours to check the next time. Some of this settings could be configurated different, I posted the default values of the avast4.ini file.
If 'all' your traffic is avast pinging their servers... well, you're not browsing that much, are you?