After cooking a pot roast in a slow cooker - how long can you leave it on a 'warm' setting before ......

The general rule of thumb is that the time the food spends between 140 F and 90 F should be no longer than two hours, total. Anything below this temperature range, which is often called the danger zone, bacteria can multiply at a very fast rate, doubling in number about every 20 minutes. If you keep it at a safe temp, 140-120 F it's perfectly safe to eat, however it may become dryed out. I usually try to limit my warming time to less than 3 hours, for parties.