The time it takes to cook beans fully in a slow cooker is between 6 to 8 hours depending on how many beans you're cooking.
A smaller slow cooker may fully cook the smaller amount of beans in 5 hours while a larger slow cooker with more beans may take up to 8 hours to fully cook and become soft.
The beans are fully done when they're soft and not crunchy.
I just take a spoon and after 5 hours I'll take one bean out and eat it to see if it's soft enough if not I let the rest of the beans cook for another couple of hours and test again.
Beans should at least be fully cooked using a slow cooker within 8 hours.
Always check on your slow cooker when you're awake or near it to make sure that water is in it and it hasn't dried up or evaporated.
I sometimes have to add some extra water to the slow cooker when cooking beans.