I have a green card, and my first re-entry permit that expires this month, I have decided not to return to the US to apply for my second one, and therefore let my green card go.
I am curious though- when do I actually stop being a permanent resident, or has that already happened? I have a 10-year green card that expires next year.
Whether you have abandoned residence is a subjective determination that looks at many factors in your situation, and it is only determined when you try to enter the US as a permanent resident. If you don't try to enter, and don't voluntarily relinquish your permanent residency by filing I-407, there is no way you can know for sure. Even if you have been outside the US for more than 1 year, without a re-entry permit, or after your re-entry permit has expired, it is still theoretically possible, after no matter how many years, for you to be given an SB-1 returning resident visa if the consulate determines you have not abandoned residency. Or, even if you have been outside for more than 1 year without a re-entry permit and without an SB-1 visa, you can still try to travel to the US (with an unexpired or expired green card), and try to enter, and it is still theoretically possible, after no matter how many years, for the immigration officer to in their discretion waive the documentary requirements and admit you as a permanent resident anyway if they think you haven't abandoned residence, or if they deny you and put you in removal proceedings, the immigration judge can admit you as a permanent resident anyway if they think you haven't abandoned residence. No length of absence from the US, expiry of documents, etc., can absolutely mean you have abandoned residence. (And conversely, even if you return within 1 year or have a re-entry permit, you could still potentially be found to have abandoned residence.)
In terms of taxes, however, it is clear -- you continue to pass the "green card test", and thus be a resident alien for tax purposes (which means your worldwide income is subject to US taxes no matter where you live) indefinitely until you either voluntarily relinquish your permanent residency by filing I-407, or you have received an official determination that you have abandoned residence. Even if it is highly unlikely for you to be admitted to the US as a permanent resident, as long as you haven't voluntarily relinquished or received a determination of abandonment, you are still a resident alien.