The green card isn't citizenship, you lose it if you don't live in the US. It's not like they don't know when you enter or exit the country.
So much has happened since then...
If you get a green card and leave the us for any amount of time, on return the border agent makes a determination on the spot if you intended to live abroad.
Less than six months is simply less suspicious than more.
If the answer is yes, well then it is yet more proof that the US immigration system operates basically extrajudicially just like the IRS and ATF, and only occassionally do the courts pull them back in after much hardship for the plantiff.
Words and policies are supposed to have meaning, and I doubt we'll get any charts or graphs on border refusals per amount of time spent abroad for GC holders.
Green cards are for people who intend to continuously by resident in the united states.
If you go home for 3 months and get a job and rent a house then you no longer continuously reside in the united states.
It's that's simple, but there's no hard or fast rule on how many days.