How long to cook a 2.500kg bit of beef in a slow cooker?

For a 2.5 kg piece of beef in a slow cooker, cook on low for about 8 to 10 hours or on high for 4 to 6 hours. The exact time can vary based on the cut of beef and the slow cooker model, so it's best to check for tenderness and doneness. Ensure the internal temperature reaches at least 60°C (140°F) for medium-rare. Adjust cooking time as needed for your preferred level of doneness.