Whilst fish can just "survive" high levels, words like "survive" and "get away with" are terms you don't want to be aiming for when keeping a pet.
There is a tank in a hospital I work in that has NEVER had the gravel vac'd in over 2 years, there is a layer of poo everywhere the tank must have nitrate through the roof. It is 20 gallons and has a red tail shark abotu 6 inch which just sits hiding under a rock hardly moves and a few guppies - they have poor colouring. I wouldnt be surprized if it was living hell. It was donated to the hospital as a gift. I think im going to take my siphon along next week and see if i can help the poor fish.