Cooking pearl barley takes around 8 hours in a slow cooker.
Place the barley in the slow cooker and then cook the barley in the slow cooker on high for at least 8 hours.
The pearl barley will be fully cooked when the barley has become tender.
The good thing about cooking in a slow cooker is that you can place the barley in the slow cooker, turn the slow cooker on high and then go do something else or even go to sleep and wake up to the barley being cooked.
After 8 hours in the slow cooker or even after 12 hours or as long as you sleep the barley should be done and it won't overcook in the slow cooker.