How long does coffee last ground / beans?

For roasted coffee in an unopened package, the best before date is about one year after roasting, and after grinding we would mark six months. However, coffee is a fresh product, so its taste changes all the time – more if it interacts with air. It depends entirely on the accuracy of the sense of taste when each person even notices the difference, and when the noticeable difference is so great that the coffee would no longer taste good. For my taste, I could notice the difference in about weeks, but good coffee would still taste good even after grinding it six months.