If you want to keep your coffee tasting better for longer, we highly recommend investing in a burr grinder so you can grind your favorite coffee as you need it. You can find a decent quality grinder for around 40 dollars. This investment will improve the quality of your coffee more than any other equipment purchase. However, if you don't have a grinder, we can grind it for you at no extra charge.
Your optimal grind level is largely determined by the brewing method you use. Below are a few of the most common brewing methods and their associated grind levels:
French Press: Coarse Grind
Drip Brewer / K-Cup / Pour Over: Medium Grind
Be aware of the time water is in contact with your coffee grounds. This time dictates your grind level. For instance, in a French Press, water contacts the grounds for 4-5 minutes which gives it the ability to extract a large amount of the contents of the ground. Machines that use K-cups can have a tendency to push water through the grounds too quickly, resulting in thin cup of coffee. While you can't control the water pressure of those machines, you can choose a finer grind to help slow the water down. On the far end of the spectrum, an espresso typically allows water to be in contact with the grounds for only about a second. To get the same compounds to extract requires a much finer grind.
Over-extraction can occur when you use too fine a grind in a brewing method where water stays in contact with the coffee grounds for a long period of time. This tends to result in bitter coffee.
Under-extraction occurs when you use a grind level that is too coarse for your brewing method. This typically results in coffee that is thin, and sour.