Usually people say its 12", at least that's what I have heard from many people.
But this isnt always true. I have sat in vehicles that have components mounted in kickpanels or the doors, with the tweeter right near them...and it sounded like crap. A week later, they came back with them installed in the A pillers and it sounded much better.
What Im getting at is that installing the tweeters right near the midbass driver is not always the correct way to do things.