There are varying opinions as to when you should clean and if you should replace filter media. Each aquarium or pond will need different attention based on how many fish are kept in there, how much waste they produce, the weather and time of year. It is best to keep an eye on your own filter and to assess how much dirt is collecting, and then to decide how often you need to do this.
Many aquarium keepers rinse their filter sponges when undergoing a water change, others will wait 2-3 weeks and then rinse their sponges only when they need washing. Pond keepers are the same, some pond owners will rinse them weekly when checking their equipment and undergoing maintenance. However, no matter how often you clean the sponges, it is vital that you do not wash them in tap water and use your pond or aquarium water to retain the good bacteria so you do not need to cycle your water again.
In regards to replacing filter media, this should only be done when you feel that it is absolutely necessary. You might notice that your media is looking very dirty or damaged, or that your pond filter is not performing to its best ability.