In general, as soon as you open the bag of ground coffee and expose it to the air, it stops being totally fresh and starts to deteriorate. Exposure to the air can cause oxidation, elements and other odors can start to infuse into the coffee, and it can lose its potency and flavor depth over time as well. This is why many people like to buy coffee in vacuum sealed bags or keep it fresh in airtight containers to preserve the freshness as long as possible, and this is the best way to store ground coffee at home. You can keep freshly ground coffee for a long time, but it will become less fresh with every passing week.