A couple of general comments from this thread.
Aside from feeding the fish, nothing is so important as water changes for an aquarium. You can never even come close to what occurs in nature, so the more you do the better. It is true that each tank is biologically unique, and this is determined by the tank volume, the fish (number, type, behaviours), live plants if any, and the general environment (decor). But regular partial water changes are the key to keeping the aquarium stable and balanced biologically, since most of us do not under-stock but probably overstock.
Get into the habit of doing a weekly water change. Set aside a time that you will have free every week--example, Saturday or Sunday morning if you work or are in school. Stick to it, always. A day either side will not hurt in weeks where something out of the ordinary occurs. But get into a habit. The volume can depend upon the tank specifics mentioned above, but more is always better than less provided you have reasonably similar parameters between tap and tank water. And doing more water changes will usually keep these closer.
Someone mentioned 25% weekly versus 50% bi-weekly being the same--they are not at all similar; there is no question but that the fish in a tank with weekly water changes will be healthier than those in one with a larger bi-weekly change, all else being the same. As an example, a 50% water change performed once a week is far more benefit than a 10% water change performed every day.
Tests are not a reliable guide to the need for a water change. If the pH begins to fluctuate beyond the normal diurnal variation, or nitrates rise even minimally, this means a water change is overdue (or the tank is overstocked or something else has occurred). But even if these tests indicate no changes, the water change is still critical. You cannot test for "crud" that is in the water--dissolved waste, urine, fish pheromones, plant chemicals, etc--and no filter removes any of these. Only the water change.
And as for tank water stability--the more regular the water changes, the more stable will be the water, and more likely to remain so. Of course, if your tap water is significantly different from the tank water with respect to hardness and pH [and this will normally only be the case if you are deliberately altering the tank water for a reason, or if you neglect regular water changes or are overstocked], then either the tap water has to be adjusted or smaller volumes carried out more frequently may be necessary, depending upon the reason.
I have heavily-planted tanks [as someone mentioned, plants allow you a bit more leeway] but I change 50-60% every week without fail, and have for 15+ years. I see positive reactions from the fish every week, so I know this is beneficial. But this is still not even close to what these fish would experience in their natural habitats.