I get 20 hours out of one load in my WSM. If you mix in a bunch of chunks with the charcoal you get smoke & perfect temps for 20 hours without touching it. I can go to sleep at night & when I get up in the AM the temp is right on & she's still smoking. The WSM will hold temps very well on it's own, but if it gets windy or cold or starts to rain the temp will vary widely. The Guru takes all that into consideration & adjusts the air flow so the temp stays steady. As I said previously IMHO consistent temps are critical for consistent BBQ.
I wasn't really a big fan of leaving a bunch of unlit lump in the cooker but when I left some is when the Guru got me 3 hours. I was just prelighting the coals and then adding them. All that's over now with the stick burner. I'm selling the Guru with the BB Chef to a buddy.