Well I keep fish stocking levels very low for a 6ftX2ftX2ft aquarium setup. I've done this on purpose to help avoid having to do regular water changes like you would have to do if you had high fish stocking levels and was puttings lots of food in the tank, generating lots of fish waste.
Filteration is also another key factor. I have two very large cansiter filters that turn the tank water over about 4 times an hour, and I even have a Juwel jumbo filter that's not been installed yet (just trying to decide if the current from it will be a problem right now), as Discus like calm conditions, and not river conditions.
Heavy planting and then feeding the plants so they don't rot and pollute the water will also help remove stuff and keep your water in top condition.
But another key factor for me is feeding. All my discus eat TetraMin, DiscusBits (same as tetramin really) and flake food. Thats all I feed them and only feed what they will eat on the drop. Apart from flake food that I hold till it sinks to the bottom at the end (which they clean up in 5 minutes). And small left overs are eater by the tetras and corys etc.
By doing this and avoiding bloodworm, beefheart and other frozed foods. So I can control the pollution levels better. So foods not left on the bottom rotting away.
As far as plants go, well I'm no expert on plants by far. Basicly I have a timer to put the air pump on at night to give the plants more CO2 when the lights go out. Plus I use TetraPlant to feed them.
You will hear horror stories about discus, they are not a begginers fish to keep. But in my opinion they are the best tropical fish to own, and can be very funny to watch as youngsters. My two blue diamons fight over flake food. Really funny.
Your answer to the water changes. Because the tank have plants in and I'm feeding them one a month. I plan to do water changes about once a month over a 2 day period (25% one day - 25% the next). Maybe even less. All depends how the fish seem.
If I was breeding and growing them on in bare bottom tanks with sponge filters like Huey Hung, I would be doing daily water changes