Lean meats (chicken or pork tenderloin) will cook faster than meats with more connective tissue and fat (pork shoulder). Bone-in meats take longer to cook than boneless cuts. Most dishes take approximately 7-8 hours to reach the simmer point on "Low" and about 3-4 hours on "High." Once the vegetables and meat start to simmer and braise, flavors will infuse and meat will become tender and ‘fall off the bone.‘
According to the USDA, bacteria are killed at temperatures above 165°F. Follow the recommended cooking times and keep from opening the lid often, especially during the onset when the slow cooker’s heat is ramping up. If you need to open the lid, remember to add additional cook time if necessary to ensure food is cooked and tender. Large slow cookers, 6-7 quarts sizes, may benefit with a quick stir during cook time to distribute heat and promote even cooking.
If your food isn’t done after 8 hours when the recipe calls for 8 to 10 hours, this could be due to voltage variations which are commonplace everywhere; due to altitude or even extreme humidity. You will learn through experience whether to decrease or increase cooking times.
You can cook frozen meat in a Crockpot™ Slow Cooker, but suggested cook time may need to be increased. To ensure meat is cooked through, use a meat thermometer. Meat should be well above 165°F to be tender.