Around 15-20%. However the interview process is quite rigorous. Make sure you are well prepared. A high number of interviewed applicants are admitted, but getting an interview is no guarantee of admission at all.
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hey! I had an interview, passed and got admitted. PM me so I can help you out maybe even send you my presentation.

Points:

1) HEC is amongst the most selective in the world. Forget the rankings just look at CAC CEO's. HEC is the best MBA in the world for French speaking countries. French companies prefer HEC to INSEAD in many cases. It also rates, amongst the top 5 in the WORLD for marketing. Kapherer is a world renowned professor.
2) Presentation topic not important. Presentation skills are the issue. Simple subject, show you know the material and really really impress.
3) Be relaxed yet professional. HEC is a rather formal school and the French take pride in this. I wore a suit and tie to the interview although in retrospect business casual might have been better with alumni.
4) Have very clear career goals.
5) My interviews lasted 2 hours each because we talked about a lot of things. As long as the conversation flows do not worry
6) My second interviewer didnt ask me any of the "normal questions". In fact, we discussed the state of the European debt crisis and Greece. HEC aims for students who are well rounded and understand what is happening in the world (of course every top MBA wants these type of students). I would advice, that you have a very good knowledge on the EU crisis news.
7) Know the program off by heart. Know every detail and club you want to get into. This is VITAL. I saw my interviewer at a MBA fair the day before the admission results and he commented to the HEC admissions officer how well I knew the program and how impressive it was. On the other hand he noted, another applicant did not and this was "alarming".