http://lawandborder.com/risk-abandoning-green-card-abroad-6-...
Key things: 6 months absence can be a factor in challenging your status as a Lawful Permanent Resident. 12 months absence means your green card (the actual document) is no longer valid for re-entry but it doesn't automatically make you a non-LPR.
Customs and Border Patrol can deny you entry, as we've just seen, on the basis executive action, legally questionable or not. But they can't take away your LPR status - only an order from an immigration court judge can do that.
A lot of this is cold comfort to any LPR caught up in this and unable to re-enter right now - but there is a process and the status itself is not so trivial to take away. The call to help organizations who provide legal aid to such people (and refugees and other visa holders) seems like the right thing to me, it's probably the most immediate need.
If you are absent > 6 months you could end up with your green card status listed as 'abandoned', but even if you come back every 6 months that is no guarantee at all that it will not be listed abandoned.
The golden rule seems to be to be more than 6 months per year in the USA, preferably substantially more than 6 months to avoid ambiguity.
In a time of grave concern for many green card holders, it's probably better to avoid this sort of inaccurate info. It's freakout time but let's stick to freaking out about true things.