To get the best flavor in your cocktails, it is always recommended to squeeze your own grapefruit juice, as with all other citrus fruits. When you buy grapefruit juice in the supermarket, it may have added sugar or have been heat-treated in various ways, which can affect the taste.
The optimal time to use freshly squeezed grapefruit juice is within 3-12 hours after pressing. After 36-48 hours, the juice starts to lose its flavor and freshness, so it is best to use it within this time window to achieve the most sublime flavor experience in your cocktail.