This question is completely dependent on the aquarium, the fish and the goals for the aquarium. Water change frequency and volume is dictated by the level of nitrate in the water and how fast it rises. For example, some planted aquariums maybe able to go three weeks without a water change, but a well-stocked aquarium with no plants may need to have a larger water change performed weekly to keep nitrates at an acceptable level. Most aquariums do well long term by having a water change performed every week or every other week and having between 20-50% of the water changed.
By doing your research about the fish you are looking to keep and making sure you do not overstock your aquarium you should be able to find a balance of what works for your aquarium although it may take a bit of time. If you have a planted aquarium with cardinal tetras and it is under stocked your nitrate may remain low for over 4 weeks! But remember if you have plants or coral, they are growing and using up nutrients within the water and after a period they can suffer from a nutrient deficiency, always consider adding a fertiliser to a well planted aquarium even if you do perform regular water changes. Planted aquariums usually benefit from at least 50% water changes to really make a big difference when it comes to replacing the available nutrients for the plants.
