We entered MGM yesterday for the first day of our annual trip with our now 16 year old autistic son. The CM asked if we are frequent visitors and offered that she could issue it for a two month period.
We have annual passports and do go several times per year. Sometimes when the CMs see that, they will issue a GAC for a longer period of time. Usually, they will only issue it for your length of stay, but sometimes they do it for a longer time.