I only gave 30 ppm as an example, usually in planted tanks you want to maintain a constant Nitrate level for plants from about 5-20ppm. Assuming a small buildup and test range amounts (Most test kits only list 20 & 30) that would be your uppermost amount you wish to have and thus perform a water change. A 50% would reduce your amount to 15ppm (or around that amount depending on a variety of factors) . A more rapid buildup might signify needing to clean filters of detritus or an accumulation of larger wastes in the aquarium. The time frame for water changes is really inconsequential. Many breeders perform 100% daily. We keep many puffers and their sensitivity to excessive Nitrates, especially in a planted tank enviroment can be a very thin line and managed carefully. Water changes are very good for the livestock in the aquarium, but performing too many can reduce your Nitrates and cause algae outbreaks and plant defeciencies.
"In the land of the blind, the one-eyed man is king" Desiderius Erasmus