Typically, homemade coffee syrup will last for up to two weeks when stored in an airtight container in the fridge. For optimum flavor of your coffee syrup, try making a smaller batch that you can use up within a week or so. This way, you can enjoy fresh-tasting coffee without worrying about whether the syrup has gone bad. And of course, feel free to experiment with different flavorings and sweeteners to find a combination that you love. Happy sipping!