To reduce water changes, you can put your turtle into a huge tank, net debris every day, or feed the turtle in a separate container. All this will help, but is either expensive or doesn't actually reduce the amount of time you spend cleaning turtle poop as opposed to taking your turtle for a walk on the lawn. To really make an impact on how often you need to pull out the hoses and buckets, consider installing a filtration system for your water turtle tank. This article tells you about filtration for turtle tanks based on my own experiences. Mechanical filtration sucks in and traps debris.
Biological filtration breaks down harmful organic components into harmless ones.
Bacteria that live in the filter medium do this wonderful work. If you want to learn more about this, a good book about aquarium maintenance will give you all the details. One aspect of water maintenance in your turtle tank is to know what the water quality is. I recommend that you purchase and aquarium test kit for that purpose and use it frequently until you understand the water in your turtle tank and how it changes over time. The most important measurements for turtle tanks are the pH value and the ammonia values.