Rich simple syrup will last up to 6 months in the refrigerator (add 1 tablespoon vodka to the simple syrup to extend the shelf life even longer). It's a more concentrated sugar syrup in a ratio of 2:1 (2 parts sugar to 1 part water). Simmer this for at least 10 minutes to make sure all the sugar crystals dissolve. The higher concentration of sugar makes it harder for bacteria to grow.