Cooking hard beans in either a slow cooker or on the stove top takes around 8 to 10 hours and in some cases up to 12 hours depending on how well done you want the beans to be.
I usually just throw the beans into a slow cooker while away from home for the day and after an 8 hour workday the beans are usually fully cooked and ready to eat.
I used to cook hard beans on the stove and it usually took around 10 hours to fully get done enough for me to eat.
I would also have to constantly check the water level in the pot the beans were cooking in and constantly stir them to avoid them sticking to the bottom of the pan.
But cooking the beans in a slow cooker is much easier and you can just add the beans and then fill the slow cooker with water and let it cook overnight or when you're away from home for 8 hours.
You can sometimes speed up the cooking of the hard beans or any beans by soaking them in water overnight and then draining off the old water and replacing the water with fresh water.
By soaking the beans overnight you also reduce the gas in the beans so you fart less but you must drain the old water and replace it with fresh water before cooking the beans if you do soak them overnight.