This is also why I actually like Slaanesh as a character and one of the Chaos gods. Each of them have a "good" and "bad" side, such as Khorne's sense of honor but also his meaningless bloodthirst. Slaanesh, though... Slaanesh just wants everyone to enjoy themselves, and through that, empower her. Every bit of joy felt by a parent when looking at their child, or the happiness of some one reveling in excess, or the loss and sadness and love felt towards a dead loved one, all of it is felt by Slaanesh.
More so than the other gods, even Nurgle, and especially the Imperium, Slaanesh is the embodiment of love and happiness.
Adding more to my previous points, it seems as though anything could be made to be un-excess. There must be many followers of Slaanesh who prefer only simple pleasures, and anything above such things would be considered excess, even if it was common or trivial by the standards of most.
To answer your orginal question of how much does it take for tame shit to be excessive?
There is no true answer to that.