I bought two 20 lb bags back in '05 & they lasted me 4 years. This was before I improved my relighting method.
Back then, every time I shut the stove down I'd clean it out completely & start with a whole layer of charcoal across the grates. Now I leave all the crap in there, put a couple layers of coal across it, 2 handfulls of charcoal in the center & surround it with more coal, soak in diesel, #2, or lighter fluid & light. At this rate I'll still be using my current 40 lb charcoal purchase 10 years from now.