Never. Because you have nitrifying microbes and biofilm all over the tank that aid in the cleaning of the water so I would never want to restart the tank from scratch.
You are going to get a lot of opinions on how much and how often to change water. What I've found over the decades is that the answer is.....( drum roll please....
) It depends. 
It depends on the fish you have, how much of a difference the new water's parameters will alter the old water's parameters and how often you are going to do them. This I know: Fish like clean water and stability. Clean water has 0 ammonia, 0 nitrites and 0 nitrates. pH levels and hardness levels do not define clean water. Stability is not changing the water's parameters in large amounts quickly. It needs to be done slowly. So if you have hardy fish, they can usually handle a larger change in water parameters than more fragile fish. If you change water more frequently, the fish generally handle larger volume water changes. If you only change water once a month or longer, doing large volume water changes can actually do more harm than good. Bottom line, you need to know your water's parameters and the new water's parameters to know how much of a water change to do that your fish can handle. I don't overcrowd my tanks so in my hobby tanks, I changed water ( 10%-20%) twice to three times a week for my more fragile fish and 20-25% once a week for my more hardier fish. For my messier fish, I changed water every 2-3 days and the volume depended on just how messy the fish tank was. In my hatchery, my fish got about a 25% water change daily only I had that done on a controlled system where water was changed 4 times during the day for just a few minutes each time. On some of my friend's fish farms where they had spring fed water or artesian wells, there was a constant trickle of water going into their pools 24/7. So as you can see, it all depends on what you have. There is no one answer for it all. 
