It will vary greatly depending on the scene. For instance this week I've been rendering some HD animations which have varied from 40s to 5 mins per frame depending on the scene on a 3960X, and I've certainly had HD that takes 20 mins per frame too sometimes. While I hope others can share their experience, one suggestion is grab the 45 day demo, because then you can try it out with YOUR scenes, and know how it performs with what you tend to render and how you tend to set up the scenes.