It depends on what type of dressing you have. If you have oil-based dressings and don't contain any vegetable or fruit juice, it's still safe for consumption since it contains natural preservatives that can help it last for hours at room temperature. A good example of an oil-based dressing is Italian dressing. On the other hand, you should throw away dairy-based salad dressings and oil-based dressings that contain vegetable, orange juice, or mayonnaise left at room temperature for more than two hours. Some good examples of dairy-based sauce are Caesar Dressing and Blue Cheese Dressing.