I think it depends on how much biomass is in the system. Ammonia and nitrate reach there highest concentration near 21 days in a normal setup. If the tank has a low biomass it may take longer two reach toxic levels.
One of the students here at Tech had a RAS setup with 50 2" Pompano. He didn't add any filter media until a month after the system had been running and after two months the fish started dying. The system is about 400 gallons.
