I guess I'm spoiled with my cooker.. Roughly the same size (450sq in), and 8lbs would last me 24-36 hours easy, of continuous cooking.
Rarely, do I add fresh lump more than once a week, and often I cook 4-5 times a week on it. Occasionally more.
Heck, the 20lb bag of Mali's I just bought, will probably last me 3 weeks.