You can cook it for about 4-6 hours on High or about 7-8 hours on Low, depending on the size of the chicken breasts. Some chicken breasts have the tenderloin rib meat attached, which makes them thicker and longer to cook. So take that into consideration for your cook time.You can cook it for about 4-6 hours on High or about 7-8 hours on Low, depending on the size of the chicken breasts. Some chicken breasts have the tenderloin rib meat attached, which makes them thicker and longer to cook. So take that into consideration for your cook time.See post for additional recipe details and to watch the video on making the slow-cooker miracle garlic chicken.