The interruption is a chain event... you might be aware of this? but just making sure. You interrupt her first at school, then immediately in the bath at home (you may need to skip time once?) but check the bath at home before that, then immediately after the bath interrupt her in the bedroom (so immediately go to her bedroom as the time will have switched to night right after the bath interruption scene). this brings her total frustration to 6... do it two more times to bring her frustration to 10 (so start the cycle again, once at school, once in bath =2 more times). Now with frustration at 10 you can ask her for more at school... (this lowers her frustration to 0 and libido to -5) and then repeat the whole cycle again and ask her for more in the bathroom. her libido/lust needs to be 10 for her to be interrupted.
So if at any time you acted outside of the chain... ie. timing to go the bedroom is wrong and so there is no interrupt option and she just climaxes after masturbation, this will lower her libido and you'll need to bring it to 10 again (legit or with cheats).
There is currently no frustration stat counter... (that i'm aware of?) which might be something
WillTylor might want to add in a future patch/update so people can better keep track of their frustration score. It helps to have it because you sort of expect it.. and then when it's not there you're like oh... i guess i should have been counting how many interrupt scenes i did... but i mean it's not a lot to keep track of anyway, and if it won't be an enduring mechanic then it's not urgent to have a display for it in the UI.