I got a WSM 18.5" recently and did a smoke on it this weekend, just a basic pork butt. I filled the charcoal ring and put 4 or 5 chunks of pecan in there, I'd estimate it used about 2/3 of a bag of Kingsford. I lit another 25 briquettes in a chimney and spread them across the top of the unlit charcoal, assembled the smoker and filled the water bowl with hot tap water. By 7:30 PM the smoker was at 210 degrees and the meat was on.
After that, the lowest temp I saw was 210 at 2:30 am (I had two of the vents fully closed, so I opened them up slightly) and other than that the thing ran between 220 and 235 with one spike up to 250, but I never saw a temp reading higher than 250. I never added any fuel, all I did was add a little more tap water (maybe a half gallon) when the water bowl was starting to look a little low. At 8:00 am, more than 12 hours after it got up to temperature it was still cranking along at 232 degrees.
At 8:30 AM an absolute monsoon rain storm blow in and annihilated the smoker with super heavy rain and very high winds which killed the temperature so I moved the pork butt to the oven to finish (it was about 190 when the temperature started to drop). When the storm finally passed I disassembled the smoker and there were still some briquettes in there, along with a nice pool of water/ash soup right under the charcoal grate from the rain. So, I got 13 hours of low temperature cooking and the fuel was not all gone yet.