How long to cook rump roast in crock pot, on low heat?

As a matter of fact, I have one in my crock pot rite now. I take and cut up potatoes in chunks, slice carrots in half ( baby carrots work best), and 1/2 of a large onion and place in the bottom of the crock pot. Then I season my roast with ground black pepper, garlic powder, and salt (litely). Place the roast on top of the vegetables and cover it with a can of Cambell's double beef broth or consumme. (Depending on how much vegetables, you may need 2 cans. Just do not cover the meat (only just past the vegatables). Cover and leave alone for 6-8 hours on low. That's it! It comes out great every time!!