You have six months from the time of your medical to activate your visa. So, say your medical is 1 October, but your interview isn't until 1 November, they will count the six months as beginning on 1 October. So your time to activate the visa expires 1 April, exactly six months after the medical.
Activation doesn't require you to permanently relocate. You have up to 12 months after activation to tie up affairs at home before making the permanent move. We activated our visas in June of this year but won't be making the permanent move until January 2016 as I'm finishing work off, selling the house, letting the kidlets have a final Aussie summer. It would be June 2016 before we would be considered to have abandoned our permanent residency, should we not return.
They get that we might have loose ends to tie up.
However, as soon as you activate, you must comply with all requirements of a permanent resident, including declaring income, filing tax returns (albeit Australia and the US have a tax treaty that prevents us from being double taxed) etc. Even if you come back home, you are still considered a permanent resident of the US.