Paul,
I have not yet had to add new coals for my 18-20 hour butt cooks (knock on wood). I fill my ring up FULL leaving just barely enough room to add 30 lit briquets (without coals rolling off). Add the 30 lit briquets and assemble. I've used both sand and water (not at the same time, of course) With sand, my WSM likes to run about 260 lid temp with the vents barely cracked (approx 10%) for hours. (I adjust all 3 vents equally, except if there is a breeze, then the windward vent gets closed) With water, the vents are open about 20% to get the 240-260 lid temp range. At about the 8-10 hour mark, I usually get a steadily decreasing temp which opening the vents doesn't fix. Gently stirring the coals does fix it and I'm back to small vent openings (about 25% or more open at this point) 3 or 4 hours later, I typically need to stir the coals again to keep the temps steady and my vents are 50% open or greater to keep the desired temp range. At this point, it usually looks like there aren't much coals left but there is still plenty of energy left to finish the cook and stirring (gently) keeps them going.
Thats my experience with my WSM. Stirring the existing coals, rather than adding new ones keeps the cook going. The only time I have had to add more coals was after about a 20 hour butt cook when I wanted to cook some ABTs. I took the butt off and added the ABTs, but there just wasn't enough energy left to cook them for a hour so I added some more lit coals and finished the cook.