Supplies bought "x" months (White Giant)

number of months's worth of supplies purchased by corporations with auto buy supplies.

example;

chemicals monthly use 500 then 20 months worth of chemicals is 10000,

where do I set this feature in game?

I am aware of where to set for country supplies. But I don't believe it is possible for corporations.

If this doesn't exist, it should!

It should but it doesn't. The best way is two contracts. The first for the basic requirements, and a second for like 24 months with whatever amount you want to have extra in the corp. This second contract should have a definite expiration time.

You can override the auto buys, but it is not automatic. I sometimes check my corp orders (seperate page shows all corps and orders in one go) and then go through cancelling FMU and stuff and issuing manual orders for 100Q at 60 month max.