The timing of a slow cooker roast depends on a multitude of things; like size of the roast, type of slow cooker, starting temperature of the roast, and desired doneness of the roast. The best way to know if it is done is by using an accurate thermometer. You can use an instant read or a probe thermometer to monitor for doneness. If you want a medium rare rare roast that you plan on slicing thinly across the grain, shoot for a temperature of around 135℉/57℃ to 140℉/60℃. If you want a fall-apart tender roast, shoot for a temperature around 200℉/93℃ to 205℉/96℃.