Like regular ovens, a slow cooker can be a little faster or slower than others depending on its brand, make and model. It can be difficult to gauge the exact cooking time your roast requires until you get accustomed to cooking with your particular appliance. Even in a slow cooker, you can cook your roast beyond what's necessary.
How a Slow Cooker Works
Slow cookers gradually bring your roast up to a simmer point. If you set the temperature on low, it will take your roast a greater length of time to reach this point. On high, it will reach it sooner. After your slow cooker reaches this simmer point, the remaining time your roast needs depends on its weight and the cut. You can't overcook it up to the simmer point, but it can happen if you cook it too long after it reaches this point. It will begin drying out, regardless of how much liquid you've included in your recipe.
Recommended Cooking Times
In general, a 3- to 4-lb. beef roast will need to cook five to six hours at the high temperature setting, or up to about eight hours at the low temperature setting. Various models may recommend specific cooking times depending on the cut of your roast. The instructions that come with the appliance usually include guidelines. If you surpass recommendations by too much time, the texture of your roast will be affected.
Result of Overcooking
Leaner cuts of meat will still be sliceable when cooked to the right degree of doneness. Fattier cuts will fall apart when you insert a fork. If a leaner cut of meat begins to fall apart when you touch it with a fork, it's probably overdone. This method of testing with a fork might not tell you when a fatty cut of meat has cooked past its optimal point, however. You may not notice the difference until you eat it. A fattier cut might appear to become more tender the longer it cooks, but this is actually not the case. It will slowly begin drying out the longer you cook it. It will become stringy and more difficult to chew.
Some slow cooker brands come with built-in mechanisms to prevent you from overcooking your roast. Some models automatically switch to a warm setting when your roast finishes cooking. Others might include an "auto" setting. At this setting, your roast will cook at the high temperature for a set number of hours, then automatically switch over to low for the remaining cooking time. You can check your roast for doneness periodically. As soon as your roast is fork-tender, remove it from the slow cooker.