The short answer is as soon as you know, “more likely than not,” what the cause of the food poisoning illness is. However, that is not necessarily as easy as it may sound.
There are hundreds of bacteria, viruses, toxins and other contaminates that can cause the common symptoms of food poisoning or worse. Depending on the offending “bug” the onset of symptoms can be hours, days weeks or months. For example, consumption of food tainted with Norovirus (most common food poisoning illness) has an incubation period of a few hours to a day, and Listeria (one of the deadliest food poisoning illnesses) has an incubation period of three to seventy days.
The key is to get prompt medical treatment and to get stool and blood tests to determine the bug that is causing your illness. Most of the common illnesses like Norovirus, E. coli, Salmonella, Listeria, Hepatitis A and Listeria are reportable by the hospital to local and state health official who are tasked to investigate to cause and source of your illness.
So, my advice as to when to file a claim is when you know, “more likely than not,” what the cause of the food poisoning illness is. In my opinion when to file a claim is when you know what the bacteria, virus or other contaminate is that has made you ill and you can positively link it to a food item or location.