So you have landed a 457 visa sponsored by an employer. How easy it it to convert this to a permanent Australian visa.
Alot comes down to your employer. They have the possibility to sponsor you any time they want. Even from day one.
So you employer could put you forward for the permanent 856 visa straight away. However there are special conditions surrounding this. For instance computer experts must have at least four years work experience in their specific field either in Australia or overseas to get passed experienced by the Australian Computer Society.
If yoiur employer will not put you forward for the 856 visa then you can apply by yourself or pursue sponsorship from one of the states. The criteria without an employer sponsor is much stricter though.
If you gain Australian permanent residency it is valid for five years. You can quit the country and come back any time within those five years. Once inside you can stay for as long as you want even if the permanent residency has expired.