This largely depends on your management system which includes your stocking density, the type of water you are using and the type of feed you are giving to your fishes. High stocking density will get the water dirty earlier because the more they are, the more the quantity of feeds and feed wastes and also the more the wastes from them so high stocking density will necessitate regular change of water for the fishes.
In addition, contaminated source of water and local feeds get water dirty easily and so will call for regular change of water. Generally, water in a fish pond should be changed at least once in two weeks and also as soon as the oxygen level of the water is low. Therefore careful monitoring should always be carried to ensure that the water oxygen level is maintained in its good quality as nothing kills the fishes faster than bad water.
Ensure to change your water immediately you discover any abnormal signs from your fishes; do not wait for later as every minute counts in fish farming business. You “LATER” may end up becoming your “LOSS” if proper care is not taken. Therefore protect your investment.