You need exactly 1/3+2lines/7pagesX8letters of dialogue per 2/3 of description in your book.
Okay, just kidding.
It really does depend on the book, genre, characters, and your personal style. If you're dialogue-writing is good, then I suppose you'll want to play with that, but the main thing to remember is not to force it (no matter how good it is). If the characters should be talking, then dialogue is applicable. If it's an intense, quiet moment, don't feel you have to force some dialogue in there (unless you have a character who wants to break the silence to ease the tension).
Try joining a critique group (private with password protected to ensure no copyright issues) to get specific responses to your dialogue and how much or little there is.
-D