While I agree that link building should start as soon as possible, I think it's also important to clearly define when that is. The point that I would begin a link building effort would be after you have enough content addressing your target keywords on site and specifically in the form of blog articles, how-to guides, resource information, etc. These pieces of content need to provide informational value and typically deliver some sort of "how to" or "this is what you should do" approach to the content. This will ensure your link building campaign will work that much better. Good luck and let me know how I can help!

-Jeremiah