Salad dressing is perishable and can spoil if left outside of the fridge for too long. The general rule of thumb for perishable foods, including salad dressings that contain ingredients like mayonnaise or dairy products, is the "2-hour rule." This rule states that perishable foods should not be left at room temperature for more than 2 hours.
Bacteria can grow rapidly at temperatures between 40°F (4°C) and 140°F (60°C), and leaving salad dressing out for extended periods within this temperature range can increase the risk of foodborne illness.
If the ambient temperature is particularly high, such as during hot summer days, the 2-hour window can be reduced to 1 hour to ensure food safety. If the dressing has been left out for longer than the recommended time, it's best to discard it to avoid potential foodborne illnesses.
Remember, if you're at a picnic or outdoor event, keep the salad dressing and any other perishable foods in a cooler with ice or cold packs to maintain a safe temperature.