My systems have normally run fairly high nitrate levels but in the winter when the fish don't eat as much, my nitrate readings do normally drop down into the orange.
Even after removing the bananas, a week of the fish not eating much quickly brings my nitrate levels down into an easily readable range and I suddenly don't feel like my system is too heavily loaded anymore.
I expect that if I didn't have the washed shells in my system (and therefore the high pH) I would probably struggle more with lacking nitrate levels during the cold snaps rather than the high nitrate levels through most of the year but my system does have enough solids now constantly breaking down in it that I suspect that I could leave it to run fishless for quite a long time and still get readable nitrate levels for a long long time. The only real drawback from leaving a system fishless for a long time would be the need to cycle it back up to a heavier fish load when getting fish again since the ammonia coming from the slow break down of solids is not going to be much compared to an actual fish load.
But rest assured, the system still ticks over even at the colder pace of winter here in Florida. The ice only killed off the warm weather plants and there are plenty of veggies that do just fine in the cold. As long as the water doesn't freeze in your pipes or block the flow into the grow beds, a system can keep on going through the cold.
Now keep in mind that if you normally keep things heated and you get a sudden freeze due to power out or other mishap you may need to carefully watch the nitrite levels if you warm things back up un-naturally quickly as water below 50 F can really put a damper on the bacteria that convert nitrite to nitrate. So watch the ammonia and nitrite levels as you warm things back up and ramp feeding back to normal. In natural systems the warming and cooling generally happen slowly and give the bacteria colonies time to catch up to the changes.