Maximum metal-metal distance for antiferromagnetic coupling
The annoying answer is that this will be system dependent. I'll try my best to explain with a quick recap of magnetism.


Unpaired electrons produce a magnetic moment
One of the factors that determines the strength of the magnetic moment is the number of unpaired electrons.


If you have a specific system in mind, I would suggest performing a series of single-point (no ionic relaxations) calculations where you increase the distance between the ions. Contrast the energetic between FM and AFM configurations. If you have a license, VASP can provide information on the magnetic moments.