I have never been issued that 7-day....but I believe they add it to whatever your existing date stamp says. Just note that once you get the 7 days to exit stamp in your passport, you don't have ANY options anymore other than leaving. So don't take that stamp if you think you might have some other options (e.g. student visa, marriage, retirement, etc.). But those take time to do, which is why it's best not to wait until the last day.
From what I have read, it seems to really depend on the temperament of both the IO at the airport desk when you arrive, and also your own. (E.g. be respectful, dressed decently, etc.). It is also much better to have a TR visa rather than entering using the visa exemption program. IMHO