... How do you know it isn't checking every 120 minutes, e.g. where are you looking ?
Good point ... there's no notification (nor any need for one, that I can see) if it's checked and nothing new is available.
For whatever reason (probably at least partly my rather skimpy RAM), nearly everything slows down on my system when avast.setup is doing its thing, and in the case of such a slowdown I can usually assume an update check is in progress, or even double-check that with Process Explorer.
If PE (or the default process manager, or whatever) had an option to keep a log of process events, you could check the log to see how often setup was active. Admittedly, the log would grow
very rapidly to an incredible size, given the nature of computers, and I sure wouldn't want it.