Homemade tonic expiration date

I hadn't really experimented with my homemade tonic water (or consumed much of it either) since I made it on May 31. Today I was thinking about tonic again and decided to have a look at the bottle in the refrigerator. Bad news- it had mold growing on top. (It was in a whiskey bottle with metal screw cap that I repurposed to hold the tonic water syrup.)

So for all of you wondering how long your homemade tonic syrup will last in the (my) refrigerator, the answer is less than a month.