slow cooker pot roast with carrots and potatoes
If it's a joint of pork, you shouldn't - they have to be cooked on high to get a high enough internal temperature.

For other meats most of my books say you should roughly double the time if cooking on low, so 10-12 hours, with the lower end assuming a good preheat on high and browning the ingredients first. But there's no need to set a timer. A stew will keep well indefinitely on low, and this will stop it cooling down too much if you're delayed.