Different rainwater harvesting systems will necessitate different levels of cleaning. For example, a system made for potable water will require more cleaning than a system made for watering plants. In addition, a dirtier location will need more cleaning than a clean location. Finally, a system with a well-designed screen and first-flush will require less storage cleaning than a system without these measures.
The most common cleaning methods include visually inspecting gutters and screens often for buildup that can be cleaned by hand. Checking every month for the first year will help you determine what schedule you should be checking at in the future. In addition, the storage should be checked every year or two for buildup. Buildup should be removed by hand, vacuum, or a drain (if one is designed into the system). After cleaning out buildup, check the inside storage surfaces for any scrubbing that may be needed. Finally, depending on your storage tank material, you may consider adding chlorine periodically for cleaning. See appropedia.org/Rainwater_tank_cleaning for more.