Hydroponics systems should be cleaned whenever the nutrient water is changed. That interval can vary, depending on the types of plants you're growing, the requirements of your particular system, and the size of your operation. Some triggering events can require immediate an immediate cleanout and replacement of your reservoir water. Since a thorough cleanout requires draining your apparatus of water, it's wise to combine it with a planned replacement of nutrient water.
Cleanliness in a hydroponics operation is important. Since plants are grown in a sterile environment, any incursion of bacteria, algae, or troublesome microbes can quickly damage or completely ruin an entire crop (or more). It's always a good idea to do a thorough cleanout when a harvest is completed and a new round of crops are being introduced. Keep in mind that cleaning or sanitizing is different from sterilization. The latter operation is more intense and requires stronger cleaning agents and techniques.
