It took me 165 spawns to get 3 casks. I saw 20 gnoll brewers, 145 gnoll guardsman, and 0 master brewers. I also killed about 10 spawns before I started keeping track, which were all guardsmen. I saw brewers on spawn number 4, 7, 8, 29, 38, 49, 51, 55 (cask), 69, 92, 93 (cask), 109, 134, 138, 142, 145, 146, 147, 161, and 165 (cask). Spawn rates are between 13.5 minutes and 17.5 minutes usually. And yes, the brewer/guard spawns around -32,-32 and then wanders over to the barrels after maybe 10 minutes. They do eventually wander back to the water as well. I also posted this information on the quest link.
There have been comments that the Master Brewer can spawn on the top level with the wandering mob (bear, gnoll, rat) being his placeholder. I killed the placeholder quite often during my camping. Maybe close to 100 times, and I never saw the Master Brewer spawn on the top level. I never saw him on the bottom either. I will probably never see him.