If you can displace most of the air from the bottles, probably at least a year or more.

But, the part containing CD-4 may die in even less than 6 months if the bottle is big and cannot be squeezed. This is the case with Fuji Hunt 5 l kit. I have started storing the developer concentrate parts in a fridge, and it seems to extend the shelf life even if I cannot squeeze all the air off. This seems much easier than playing with inert gases.