That all depends. Anywhere from a day to months - depending upon what type of food poisoning you get. There are different types of food-borne bacteria that can make you sick. Salmonella is probably the most common and the symptoms are usually vomiting, stomach cramps and diarrhoea. A mild case may only last a day or two but can be more severe. I was in hospital for nearly a week with salmonella, and unwell for another two weeks after that, and it can kill you.
If you suspect food poisoning and it doesn't improve withing 24 hours you really need to see a doctor to find out what sort it is and how to treat it.