How much do you have to prepare your reader for the story before you actually begin?

No matter what genre I am writing in, I jump into the action. With fantasy,the book cover is going to give away that the reader is to expect unusual surroundings.

In Dragonspell, the heroine is comptemplating the beautiful city across the valley. With a little dialogue, (spoken and inner monologue) we establish why she is there, her status in the society, and her mood. There is description of the countryside as she clambers up a hill to get a better look at dragons flying into the city. At the top, she's captured by grawligs and we know more about the creatures of the world and her background. All the backstory and world-building is done in bits and pieces as they are needed. 

The main goal of the passage is not to bring the reader up to speed on the particulars of this world, but to speed the reader into the story.