Many would say once a month isn't often enough, I choose to do 30-50% water change every other weekend with my snapping turtle getting 100% change due. You'd get away with doing a normal amount and now a complete water change if you have a large enough cage and a good filter. My common snapper is already 6 years old and nearly 14'' and some 25 pounds. These guys are known to produce more waste and in general can be more messy than other species. Even with filtration you need to replace the water to get rid of the nitrate.