All I was trying to say is you can learn how fast your tank builds up nitrates. Pick an absolute maximum level and see how long it takes for them to build up. That would define your absolute maximum interval between changes.
Remember, if your tap water has 0ppm nitrates and you change at the very latest when it's up to 20ppm, a 50% change would leave you with 10ppm nitrates.
It is best to do water changes as often as practical though, but it doesn't have to be religiously once a week etc if your tank doesn't build up nitrates that quickly. The smaller the tank, the more often you'll have to do changes as parameters can change quicker. Same goes for increasing bio load. Having plants in the tank may reduce the nitrates and extend this timeframe.
I have found that many people I know got out of fish keeping because they were told they have to do water changes once or twice a week and they found it too much effort. I tend to advise people that they need to learn how their system runs and ensure that they do what they can when they can, but don't stress if they miss a week if they know their tank can handle it.