It will depend on the chicken and your slow cooker's heat output as they can vary tremendously in their potency, but in general, cook time is 3-4 hours on high; OR 6-7 hours on low heat. From a food safety perspective, if you do plan to cook on low heat (overall), you should cook on high for 1 hour (and then you can switch over to low) so that the chicken isn't held for too long/too raw in your slow cooker. That 1 hour blast of high heat cooking is what's necessary to combat against potential issues. Always make sure that the chicken is cooked through to 160-165F as measured by a digital food thermometer. You must then rest the chicken for at least 10 minutes especially if you cooked it to only 160 degrees F. I have a lengthy discussion in my Sheet Pan Whole Roasted Chicken post about internal cooking temps, how the temperature rises even out of the oven or slow cooker, so it's ok to pull it a bit early BUT you then must give a bit a time for the temp to rise during the resting period.