How often you should clean your gutters depends largely on the area where you live and how many trees are in the surrounding area. A good rule of thumb for any home is to have your gutters cleaned at least twice a year.
Here in Australia, the most optimal times are at the beginning of autumn and at the end of spring. Water build-up, water damage, rust, rodents or water failing to drain from your gutters are also good indicators that your gutters may need to be cleaned.