It depends on what you mean by monetize. If you're thinking of Ad programs like Google Adsense, I personally wouldn't add anything until 200 views a day at least. I know John Chow recommends building a list and not having any ads until your site is at least 6 months old.
If you are doing linking to affiliate products via contextual links then you can monetize that way earlier, as long as it's relevant to your content.
But in the end it's all up to you and your niche. Whatever feels right for you. Just remember that in the beginning, building up your audience and content should be your primary focus.