There is no single answer but the cleaning interval depends on the cistern design and how you are using the water. A cistern for irrigating a garden might need less cleaning than one for drinking water. There are some design features that can reduce contamination of the rainwater such as screens to keep out insects and debris. Diverters to keep the first part of a rainstorm from entering the cistern can reduce dust, pollen, animal droppings, algae, bacteria, etc. from washing off the roof and gutter system into the cistern. The goal is to collect clean rain water, not water that is washing off the roof.

Even so, cisterns can be assumed to contain bacteria and will probably need treatment with chlorination or an ultraviolet light designed for water treatment, or at a minimum the water will need to be boiled. The only way to know whether bacteria are present is to have a sample tested at a laboratory.

Rainwater is naturally acidic, and in some places is even more acidic, and low in dissolved minerals.  This sort of water will tend to dissolved materials into it (corrosivity). This doesn't make it unsafe to drink directly, but it can leach metals like lead and copper from pipes or storage containers, so that must be considered.

There is more information about cisterns and their maintenance on our website http://extension.psu.edu/natural-resources/water/drinking-water/cisterns-and-springs and feel free to follow up with one of our educators if you have further questions.