What build and version of avast are you using ?
Paid versions can be set as low as 15 min's i believe but the free version can only be set to check as often as every 120 min's - setting below this limit reverts to update interval to 240 min's ( default ).
Even if checking at 120 min's on the free version you may still only recieve two and sometimes three updates per day but at the same time you are still recieving cloud definition updates which can be up to twenty a day.