I personally just change once a week and feel people should do the same for any regularly stocked tank.
In the situation where you have a really understocked tank, for example New2Beta's 10g for Master Betta, then you might be able to get away with using nitrate tests to find out how often the water needs changing.
Likewise if you have a really overstocked tank you would want to test nitrate to find out if you have to clean your tank more frequently than a weekly basis.
In any case I have heard some debate as to whether 40ppm of Nitrate is acceptable for fish... I would change my tank weekly even fi it only reached 20 ppm.