I was changing 50g out of my 240g system once a month before I got my Schuran denitrator. I've been running Zero ppm of nitrates for the past 6mths, but I've still been changing out 50g once a month.
It seems that my fish are much happier and more lively after I change the water. As others have mentioned, there are probably toxins in the water that we can't test for.
A buddy of mine who also runs a denitrator changes 25% of his water every 3 months, so maybe I'll consider extending my changes out to once everything 1.5, then 2mths.
If your nitrates are low, in the 20ppm range, then my vote would be for larger changes less frequently.