2 hours sounds (and marketing speech is always exaggerated) kinda weird especially if this is the last DLC. I guess they just have 3rd game planned so far they decided to make a small piece in between. Harley is bit player in 3rd so we need to get her tragedy out of the way.
Personally I would like if Harley got "cured" right before she dies. I doubt it will happen in the DLC though. Perhaps in the 3rd game where she has had more time to ponder everything. Would be pretty epic if Harley saved Batman or Robin only to take a killing blow herself. More emotional burden for B-man.