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 The 15-15-15 rule (also known as the Rule of Fifteens) is a coffee freshness guideline. It states: 

15 Months: Green, unroasted coffee beans stay fresh.

15 Days: Roasted coffee beans maintain their peak flavor.

15 Minutes: Ground coffee retains its optimal aroma. 

The Breakdown: 

1. 15 Months (Green Beans)

Before coffee beans are roasted, they are considered "green." In this raw, unroasted state, they are incredibly stable and can retain their optimal quality for up to 15 months when stored properly. 

2. 15 Days (Roasted Beans)

Once roasted, coffee begins to oxidize and release carbon dioxide. While it is highly recommended to let freshly roasted coffee "degas" for about a week to balance the flavors, the beans will exhibit their peak, most vibrant flavor profile within 15 days of the roast date. 

3. 15 Minutes (Ground Coffee)

Grinding beans exponentially increases the surface area exposed to oxygen. Within 15 minutes of grinding, the volatile aromatic compounds responsible for coffee's complex flavors begin to dissipate rapidly.