The sooner the better! Your distributor needs time so they can connect the power on your requested date.
In Victoria most people have smart meters, which can be remotely connected. While this means your power may be able to be connected on the same day, the more notice you give, the better. If you have a basic meter or a site that requires a physical connection, it may not be able to do a same day connection.
If you are in New South Wales, Queensland or South Australia, two business days’ notice is needed to connect your power.