Ground mace generally retains its flavor for about 6 months to 1 year after being opened, when stored properly in an airtight container in a cool, dry place. However, the actual shelf life can vary depending on factors such as the freshness of the mace when it was ground, exposure to light and air, and humidity levels.
If you plan to use 0.25 tsp twice a week, it is likely that the ground mace will retain its flavor for several months. However, it's always a good idea to check the aroma and flavor of your spices periodically and replace them if they have lost their potency.