It seems that most people advise to fertilize every other watering. It's easy to over-fertilize, so pay attention to how your plants react to feeding. For example, if your plant responds well to feeding, you can try a higher dose the next time you feed. If it seems to be nutrient sensitive, then feed less, and less often. Keep a record of when you fertilize and when you give plain water. Keep a record of how much fertilizer you gave and make note of it. Keeping a record and paying attention to how your plants respond to feeding will help you decide on whether to fertilize or not the next time you water.