She only had sex with three people: the MC, the ex-boyfriend Jack and the Professor. Since she mostly avoids Jack, it's implied that she didn't have a lot of sex with him.
Regarding the "how long it went", I meant the affair. I haven't finished the game yet, but I reach the part when she explains that after Mr. Klaus guilted her into having an affair with him, it lasted 3 weeks. We don't know how much he abused in the meantime, but either way it's extremely uncomfortable to read. I get the devs make the story they want, but things like this are very rough on the players of a game that's supposed to be about romance. This is porn at the end of the day, people watch it to feel better not worse.
Either way, in a game where the twin sister is supposed to be in love with the MC since forever, the existance of a boyfriend at all is very "lore breaking" and the existance of a rapist 30 years older is just to cause pain to the reader/MC. All in all, decisions the dev is and will always be free to make... But it still doesnt make them good decisions ahahahah