As a coffee aficionado you will already know the importance of bean selection, correctly roasted, correct water composition, correct water temperature, correct grind, correct dose of ground coffee, correct pre-infusion, correct extraction time, correct cups, correct temperature of the cups, and so on.
As the market seems determined to use so called high quality coffee beans intended for preparation by the filter method for espresso use, which invariably produces an espresso with very bright acidity that can't be roasted out without turning the bean to soot, we have spent a lot of time evaluating the significant impact that time plays on the taste of a roast. As a roast ages the brightness of the acidity fades, and the more subtle notes in the background have a chance to present themselves to your taste buds.
A good example is the Costa Rican coffee we are currently offering from the Zamorana Estate, and also the offering from the Herbazu Estate. In our view both are awful when opened at 8 days old, and while still too bright at 10 and 12 days old the improvement for every 2 additional days of roast time is not merely theoretical or academic, but significant. We would recommend a minimum rest period of 14 days for the Zamorana Estate, and personally we try to wait 18 days if we have a bag lying around here.
It's beyond our powers to 'tell' you when the coffee is at its best, mostly due to differences in personal taste, but also differences in espresso equipment and technique have a significant bearing on what tastes best, and at what age. We are of course responsible for providing suggestions & guidelines to assist you in getting the best from the coffee.
If you fall in that group of people who are purchasing coffee from somewhere that is supposedly only a few days old, yet you can't obtain a deep rich crema (when you can from other coffee), then we would respectfully suggest that the integrity of the roast date may have been compromised and its time to find another coffee roaster. For espresso use coffee needs time to rest, for the acidity to ease back, and the complexity of the roast to present itself. The brighter the coffees that have been used (which typically runs hand in hand with coffees that are regarded as 'high quality') the longer the rest period needs to be.
If you want to improve the results you are obtaining from gourmet coffee when prepared as an espresso we suggest you start paying more attention to the forgotten variable; the number of days since the coffee was roasted. If the bag has not been opened you will have no trouble obtaining a deep rich crema from beans that are three weeks old.
Over the next while we will slowly add what we think is the optimum rest period to every bag of coffee we roast, but in the meantime it is safe to use 10 days as a 'rule of thumb' if no rest period is explicitly stated on the bag.