The stronger the syrup, the longer it keeps. 2:1 lasts far longer than 1:1. The more acidic, the longer it lasts. Some Ascorbic acid, or some vinegar makes it last longer. Essential oils make it last longer (wintergreen, lemongrass and peppermint are the only ones I've tried). A clean feeder will make it last longer. If the feeder is moldy you need to clean it with distilled vinegar.