Six months. Yes, they need to file an AR11.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
By law, green card holders who stay outside of the US beyond 180 days may not be denied re-entry but may be subject to inadmissibility. If a planned travel outside the US will extend beyond 1 year, a travel permit is required for re-entry. Failure to obtain the travel permit will result in the green card holder being placed into removal proceedings for abandonment of status.
As for the change of address, they need to file an AR 11.