Short answer is not very.
Your relationship with The Mistress is ultimately a femdom relationship, but the majority of the content will 1. Not focus on her and 2. Be soft when it is focused on her. As I said in the developer tags, there is some BDSM content, which I will expand upon in the future, but that's skippable.
The main goal of the game is variety so some future girls may also have varying degrees of Femdom, but most of the true hardcore content will probably only be with the Mistress and thus skippable.