This varies according to each pond. If you have a lot of debris from over hanging trees, then you should clean it more regularly, using only partial water changes. Many pond owners leave the cleaning too late. There is more debris and therefore more work. Often, leaves are left in the bottom of the pond from Autumn. They sink to the bottom during Winter and slowly decompose. When the water warms up in Spring, the leaves begin to decompose faster, fouling the water, promoting disease and releasing a large amount of nutrients which in turn increases algae growth.
After each clean you should add beneficial bacteria and enzymes to encourage the healthy break down the remaining small amounts of debris.