The Guide can be updated whenever you want. If you leave it for 7 days, your Guide will have almost run out. If you update it every day, you will always be able to browse at least 6 days into the future. By carrying out daily updates you will also capture late schedule changes on a daily basis.
Please note that any of the PVR's that support IceTV and that have a network connection built-in will automatically update the guide while they are turned on.