How long an 84 yo Green Card holder can stay outside USA visiting family?

If it is your mom's intention to remain outside the US for 6 months or more, then help her to file for an obtain a Form I-131 "Reentry Permit" prior to her departure. That will enable her to legally remain outside the US for up to 2 years, multiple-entry, without her risking to lose her "green card" status.

If you mom plans to be out of the US for longer than 6 months, have her get a reentry permit before she leaves. That will allow her to be out of the Us for up to 2 years. I think it would be best to consult with an immigration attorney to thoroughly review the details of her travel intentions. He or she will then be able to explain your mom's options. You can find a good one in your area at the “Find a Lawyer” section of this website. Good Luck.

6 months or less unless she obtains a reentry permit. She should maintain sufficient ties to the U.S. In any case.

Any trip outside the US for more than six months is problematic; therefore, we recommend that she stay our no more than that.