Earliest I managed was day 8, the third day with her. You also need at least 8 alcohol.
Get her drunk and mess around with her, then finger her twice while she's asleep in bed.
She'll confront you, so get her drunk and she'll wake up while you're abusing her. Take the picture, finger both holes and eat her out and she'll start liking it. Peek on her in shower and risk it, follow her to her room when she flees and eventually you should be able to rape her in her sleep while she's in her bed.
I do have cheats on, it's one step above Bobby Mode. So I didn't have to worry about maintaining stats.