Kris,
I don't have trouble getting good burn times with the 18 WSm/Auber setup. We've discussed it on another thread but I will lay out my method here. It may be some small thing you are doing differently.
1. I fill the charcoal ring to the brim with KBB (very important, I'll discuss this later in the post)
2. I pull out ~15-20 briqs to light in my chimney starter (depending on the temp I want to cook at)
3. While the chimney is going, I set up my Auber fan and controls.
4. Close all bottom vents except the one the fan is on. (other holes on the fan vent are covered with aluminum tape)
5. Top, exhaust vent is wide open
6. Make sure the fan is as sealed as reasonably possible so most all of the air is going directly into the smoker. (usually is flush with the smoker but I check to be sure)
7. Once the briqs at the top of the chimney are just lighting and flames are licking around them I dump the lit coals into a small indention I've made in the top of the full load of briqs in the charcoal ring. (lately i've been experimenting with putting the lit coals inside the ring closest to the fan for better control but either way works)
8. Assemble the smoker and turn the auber on to the temp I want to cook at. (Should be using a 6.5CFM fan for an 18 WSM or a 10 CFM fan for the 22 WSM)
9. Wait about 30 minutes for the smoker to come to temp. Sometimes this will take up to 45 min to 1 hr if it's really cold outside.
10. Add a couple of wood chunks through the door
11. Put meat on the smoker and go back to helping the wife wrangle the 3 gremlins
I can get 8-10 hours no problem cooking at 250-275 with this method. (usually can get up to 12 hours in warmer weather) The key is to make sure to fill the charcoal ring TO THE BRIM with unlit coals. I usually fill it about 2/3rds of the way up with unlit coals, shake the ring to help pack the coals down and finish filling it up. This helps me get the most briqs into the smoker as possible.
On an 18 wsm i can get ~ 10-12 lbs of charcoal into the ring. (don't know exactly the amount here, just guessing)
Good luck. PM me if you have any more specific questions about the WSMs or the Auber setup