Yes, it is possible to overcook beef brisket in a slow cooker. While slow cooking is a great way to tenderise tough cuts of meat like brisket, if the meat is left in the slow cooker for too long, it can become dry and stringy. The amount of time it takes to cook beef brisket in a slow cooker will depend on the size of the cut and the temperature of your slow cooker. In general, beef brisket should be cooked on low heat for 8-10 hours or on high heat for 4-6 hours. Use a meat thermometer to check when it is fully cooked.