1. How long does it take for food to rot?
All stored food is subject to rot every day. Food is not tracked by age. There's just one big bucket of food in a region.
Or is it just a percentage of the stores in the granary?
Yes.
Food stored in a granary rots at a lower rate than food not stored in a granary. IIRC it's 1% for food stored in a granary, and 3% for food not stored in a granary. Each granary holds 1,000 bushels. For example:
* You have 1,500 bushels stored in a region with 1 granary. which holds 1,000 bushels.
* The first 1,000 bushels will rot at a rate of 1%, for a 10 bushel rot.
* The next 500 bushels will rot at a rate of 3%, for a 15 bushel rot.
* The total rot is 10 + 15 = 25.
For a city with 3 granaries and 5,000 bushels stored:
* The first 3,000 bushels will rot at a rate of 1%, for a 30 bushel rot.
* The next 2000 bushels will rot at a rate of 3%, for a 60 bushel rot.
* The total rot is 30 + 60 = 90.
* Building two more granaries would put all 5,000 bushels in a granary, lowering the rot rate to 50 bushels a day, saving 40 bushels.
It doesn't matter where food is stored, so long as it is in a granary. Spreading your food out makes it less vulnerable to monster attacks, but could be more vulnerable to looting in exposed regions, and requires more micromanagement to keep your cities fed. (You have to move food from more places, more often.) Concentrating your food in cities makes it easier to defend, and less micromanagement, but requires more granaries in your cities to hold it all.
2. If there is a finite time before food rots how can you tell which food is newer and which is older?
As above, food is not tracked by age.
