Roasting is a chemical reaction and one of the things it produces is carbon dioxide. A lot of that gas stays trapped inside the bean’s porous structure when it comes out of the drum, and over the following days it works its way back out. That process is degassing.
What the gas does to your brew
It gets in the way. Carbon dioxide escaping from the grounds physically pushes water away from the coffee, so the water cannot make proper contact. What you taste is a cup that is sharp, sour and uneven, and the usual reaction is to blame the beans.
You have already seen this happen, by the way. That is the bloom: the mound of foam that rises when you pour the first water onto fresh grounds in a pour over. That is carbon dioxide leaving. The reason the bloom step exists at all is to get as much of it out as possible before the real brewing starts.
So how long does it need?
Long enough that the gas is not interfering, and not so long that the coffee has gone flat. The window is different for filter and for espresso, because espresso is a much more violent extraction and gas causes more trouble in it.
Why we think about this before the coffee reaches you
Because the bag you buy from us should not need you to wait around. We account for degassing when we are deciding what goes out on the shelf, so by the time a bag is in your hand it should be ready or close to it.
If you have bought something straight off the roaster though, give it a few days. It is worth the wait and it costs you nothing.
Try this
Buy a bag the day we roast it and taste it three times: that day, four days later, and two weeks after that, brewed identically each time. Most people find the first cup sharp and closed, the second noticeably rounder, and the third somewhere near its best. You will have found your own rest window, which is more useful than any number printed on a bag.