The best rule of thumb is: when in doubt, throw it out!
The longer we allow potentially hazardous foods (like coleslaw) to remain in the 'danger zone' (40 to 140 degree Farenheit), the higher the risk of that food causing illness. If these types of foods remain in the danger zone for more than 2 hours, throw it out.
In the case of foods left out for serving during a meal or party:
1. Try to keep cold foods cold (40 degrees or colder) and hot foods hot (145 degrees) for as long as possible.