From what I’ve heard from people I know and understood from Aytullah Sistanis rulings, if I buy food such as grains for your house and a part of it is unused after a year, I’d estimate the value of the unused amount and remove Khums on that.
Am I understanding that right?
Is that applicable on all food or just commodities like grains,oil? and what if I bought something just last week?

Yes, this applies to everything that has financial value