There should be a Goldilocks-range for this, and what this supposed to be depends on your field, topic (whether relatively new or not), and most importantly, your advisor and thesis committee.

<img src="http://i.stack.imgur.com/YpWMj.jpg" width="400" />

([Image ref. PhDComics.](http://phdcomics.com/comics/archive.php?comicid=1821))

  [1]: http://i.stack.imgur.com/YpWMj.jpg