Well as far as i know about cooking on pot roast cus my mom does the same. You just put it at a realy low heat so it cooks very slow. Reason is it for when you dont have time to cook dinner or something so u pop it in the pot at noon or a time ur available and take it out when its dinner. Something like that :)
For a 5lb, it would take 6 to 7 hours to prepare coupled with the fact that it is on a slow cooker. This is based on the estimates that for a 31/2lb pot roast recipe it would take 4 hours to prepare so it would take longer for a larger one.