Some people advocate making glazes ahead of time, but my advice is to make it right before using it.  However, if you need to wait to glaze a cake, it can be stored in an airtight container in the refrigerator for a few days.  Bring it back to room temperature and whisk before using it. You may need to add a few drops or more of liquid or more sweetener to get it back to the desired consistency, as it may have set and hardened a bit.