Final answer:
Containers that held shellfish should be discarded 90 days after being emptied or when the last shellfish is served to prevent foodborne illnesses, especially due to potential toxins from 'red tides'.
Step-by-step explanation:
According to food safety guidelines, containers that held shellfish should be discarded either 90 days after being emptied or after the last shellfish was served from it, whichever comes first. This is important due to the risk of foodborne illness associated with seafood if not handled properly. The safety measures are stricter for shellfish because they can accumulate toxins, especially during algal blooms known as 'red tides', which can lead to serious illnesses or death if consumed by humans.