My opinon: water changes do more than just get rid of nitrate build-up. Plants can also take up ammonia and thus keep nitrates down, but they always need to be growing to do this. And only a water change will clean out other stuff that accumulates in the water and on the bottom, and replace it with new trace elements, etc.
I change water on my planted tank every week (50-80%), but it has gone as long as three weeks with no obvious ill effects.
Also, remember that the longer you go between water changes, the more likely that you will have significant pH changes. If you don't change the water often, then test the pH and compare to your tap water to make sure you won't shock the fish - or just do a small water change.
Suggestion: do water changes at least every other week, and make sure your setup will still allow plant growth as summer turns to winter.