Avast free can only update at 120min's minimal, setting to below this limit will revert it to checking every 240min's.
Oh? My mistake. Thanks for the correction, Craig. Well, two hours still better and twice as frequent than four, if that's what Matteo wants.
[edit] Hmm, are you sure, Craig? Does the readout reflect that reset to default? I just tested that quickly with a free version of Avast, and the entered value itself, at least, set to 60 minutes, remains unchanged for the moment.