With most big predatory fish, change as much as your time and finances allow. Obviously, there is a stress factor, but if sensitive fish like discus can adapt to daily water changes, our predators will be fine.
Even clean, well filtered aquarium water is INCREDIBLY filthy compared to natural bodies of water. This is true for nearly all aquariums including super high dollar low nutrient reef tanks. Think about stocking densities. The solution to pollution is dilution, and our relatively tiny tanks are very concentrated.
I've been doing 50%/day, but would like to cut back a little in part because of my water and gas (water heater) bills.