If you are granted an Australian Permanent Resident Visa it will have an ‘expiry’ date on it.
While the visa allows you to remain in Australia for the rest of your life, the date is the date up to which you can remain outside Australia and retain your permanent residence.
If you wish to extend this period outside Australia you will need to apply for, and be granted, a Resident Return Visa. These visas can be for varying periods of time and have strict criteria for their grant.
If you believe that you will need such a visa, you should seek specialist advice well before the ‘expiry date’ arrives as you may be able to conduct your affairs in such a way that you will be eligible for the Resident Return Visa when the time comes.