How often gutters are cleaned can depend on the location and the surrounding elements, such as overhanging trees. Flat roof gutters may need to be checked and cleared more often than first floor gutters if this is the case. Generally, annually or twice annually is enough to keep gutters free flowing.
It is always a good idea to check gutters during or shortly after heavy rain, when it is easier to spot a leak, damage or an overflowing gutter.