Most people here recommend doing water changes once a week or every other week.
But there are so many factors that determine how necessary a water change is, such as:
Calcium/Magnesium ions (GH)
KH for stable pH
For example if you had a large tank with only one small fish, as long as the nitrogen, GH, KH, TDS, etc levels and all other water parameters remained healthy, a water change isn't really necessary as all the parameters are at good levels. So theoretically that set up could go many months or longer without needing a water change as long as levels remained good.
Just mentioning that as some people just blindly say water changes MUST be done weekly/bi-weekly without understanding what water changes actually do, which doesn't make a whole lot of sense to do them when they aren't actually needed. Nothing wrong with fresh water though, but just saying.
Now if you have a high bioload or fish stock, those levels/parameters can get out of balance a lot easier, so then more frequent water changes will likely be more necessary.
Just as another mentioned, not all tap water is healthy, some don't have sufficient GH, KH, etc, etc, to keep water chemistry healthy for livestock, so it does get a bit more complex than simply suggesting water changes for every issue.
If you are doing a fertilizer regiment, some of those recommend weekly water changes or so, so those may be a factor that dictates water change frequency.
Hehe, sorry if I made things more confusing for you
But to simplify for you, do water changes as necessary. If your nitrates get past 40ppm, then do a water change, whether that occurs daily, every other day or weekly, will be found out based on test results. Water change once a week is a good start. Test nitrates before that weekly water change, and if it's 20ppm or so, you can probably extend water changes to every other week (which then the nitrate test would read 40ppm or so).
By the way, nitrate tests can be inaccurate and need calibrating (reference to determine how accurate your test kit reads)
Read here on how to calibrate your test kit