Push update is new feature of avast! version 4. Standard iAVS service is started on user demand so program is up-to-date only in case user take care of it.
Push update is based on e-mail sent from us to our customers. This e-mail is recognized by our providers checking your incoming e-mail for viruses and if it is received, iAVS is invoked.
So if you won't check your e-mail for a week iAVS is not started, of course. But in this case chance of infection of your computer is very small because e-mail is mostly used method of today's viruses to infect your computer.