It very much depends on the dimensions of the window you're covering and the style of the drapes. Can you post a photo?

Mine are only 8' at the lowest point, so I hang them there.

6" above the window frame.

mine are 7'' above the window frame...

Visually, they can look best the higher the better. Putting them at or brushing the ceiling can also increase their energy efficiency, by forming a seal without need for a valence.

If you have crown, I would put them just below it. Other wise I always hang mine as close to the ceiling as possible. Even if I have a shorter window. To cover the space between the window and the curtain I use roman shades and hang them on the wall and let them fall to the length of just below the window frame. It gives the illusion of a larger window and height.

If you take a photo of your window, I can do some mockups for you.

The thread linked below will give you lots of ideas on window treatments and how to hang them.

Here is a link that might be useful: Window treatment ideas

Thanks Annie, this link is really helpful for me too...

I hung mine 5" below my 8' ceilings. It might depend on how big the span is between the top of your windows and the ceiling.