100 trust to bathe with her at all.
200 trust to get the option to wash her or have her wash you.
When washing her back, the option to rub one out all over her is always available. Depending on her stats, she might catch you in the act and be displeased, or she might not realize what you're doing until you finish, or she might think you squirted soap all over her, or she might decide to take care of you herself.
When having her wash your back, 150 interest to get her to wash your front by hand, or 200 interest for her to use her mouth.
When having her wash your back, if both her interest and her lust are high enough, she'll wash your back with her boobs, and then she'll wash your front on her own.
Whether washing or being washed, 200 interest is required for her to accept H. Penetration experience (hidden stat) is also necessary for her to accept vaginal or anal; she'll flat out refuse if her XP isn't high enough regardless of her interest and lust levels, and in the case of anal she might accept at first but then scream in pain and beg you to pull out if she doesn't have enough XP.
Other H options are HJ, BJ, buttrub (always available from day 1), and paizuri (requires reading a Thin Book to unlock the skill, then 40 SP to buy the skill; will always be available thereafter)