Pouch capacity varies a lot, but it’s not like filling a bucket with water. Because the pouch is continent it will continue to stretch (ultimately with increasing discomfort) pretty much until a decision is made to empty it, for the various reasons people decide that it’s time. Diet plays a role in that varying diets can produce quite different amounts of residue (e.g. vegetarian vs. a low-residue diet). Stool produced by a colon has an extraordinary amount of bacteria (about 30%) by volume, but I don’t know the (presumably much smaller) fraction for J-pouch output. An inflamed J-pouch will feel like it needs to be emptied at a much lower volume than a healthy pouch. So: YMMV