Depends on the tank. My planted tanks stay at ~10 or less. My discus grow out stays down to 5ppm or less, my big carnivore tank stays around 15ppm, and my Achara cat growout stays at 25.
also when comparing answers many tests are actually reporting ppm nitrate -nitrogen rather than ppm of nitrate alone.......... many people are not aware of which test the are using...... the answer differs by 4.4X