How often should I change ALL the water in my aquarium?

Never unless your breaking down a tank 25% weekly minimum should be enough

Never. Changing all the water in an aquarium creates too much instabillity and fluctuation in water quality and chemistry and in consequence could kill your fish. Try to just change 5-10% of your water every week and siphon the tank with a water change of 15% once a month or bi-weekly. But everything also depends on the fish you have. But always remember, NEVER change the complete aquarium water.

Large water changes are generally not harmful to fish. However, they are completely fatal to the bacteria that are keeping the fish alive. Think of the task of caring for fish as an exercise in bacteria keeping. If the cycle bacteria aka beneficial bacteria are happy and comfortable the fish will be fine. These bacteria are sensitive to nearly any changes in their environment, temperature, water chemistry, lack of oxygen, and just about any other sudden changes. Many people think that adding chlorinated water to make up a water change is a safe practice, and if the water change is 20% or less it might be OK, but larger water changes should be "made up" with pretreated water. My feeling is that if one is trying to preserve bacteria colonies why add bactericidal water and then remove the poison?

in the twenty years I've been servicing aquariums I've never changed all the water, not even once.

that should answer your question,,

however, there have been times, like when moving an aquarium or super cleaning a tank, where I've removed and saved over half the water and all the dirty filter materials and then returned them to the tank once the tank has been moved or redecorated or cleaned.

But I've never, ever changed all the water in a tank.

note: bettas are remarkably resistant to large water changes and that's why people think it's ok, it's not and partial water changes are always better and safer.