Because cumin is a seed, it keeps its aroma for a much longer time when stored whole than when ground. The packet itself is only 20 g, so it will probably not sit in your cupboard for months, but how you store it still affects how strong the spice tastes by the last spoonful.
Keep the seeds in an airtight container — a small glass jar with a tight lid works well. Put it in a cool, dark, dry place, away from the stove, oven or any heat source. Heat, light and moisture all speed up the loss of the essential oils that give cumin its smell and taste. For the same reason, it is better to keep the container in a cupboard than on an open counter, especially near the hob, where steam and warmth can reach it.
Moisture is the factor people most often overlook. If steam gets into the jar, the seeds can become dull and lose their aroma more quickly. Scoop the seeds out with a dry spoon and close the lid again right away.
Whole seeds stay usable for a long time, but not forever. A reliable check is the smell: if the packet no longer smells clearly of cumin, the seeds will add very little to a dish, and nothing you do at that point will bring the aroma back. It also makes sense to grind only what you need for the meal — ground cumin fades noticeably within days, while whole seeds hold up far longer.