If the label on Gaviscon Peppermint Flavour Liquid does not specify how long it can be used after opening, the safest approach is to follow the instructions in the package leaflet if available. In general, it can be used until the printed expiry date, provided it is stored correctly in a cool, dry place with the cap tightly closed. If you notice any change in its colour, smell, or consistency, you should stop using it and discard the product.