Rebel Duet is planned as a 7 act story, but there's a very strong chance this will be increased to 8 acts in total, since I'd like for Hinomoto to be an entire story arc of its own.
If you mean the story branching off between the normal and corruption routes, Act 5 probably makes the most sense since the act itself is thematically focused on good vs. evil. But sexual corruption can happen much sooner than that. Act 3 will already open Valerie and the MC up to a lot of different kinks and fetishes, based on the player's choices.