I know it will vary depending on the quality of links and what have you, but how long do you think it takes for the search engines to factor in these links in terms of impacting placement.
I only have experience with a couple of sites and my approach between the two was radically different. In the first I released the entire site ready to go with all pages online and then started building links. When I think back it wasn't a very natural approach and it probably took 3 months or so to see any real improvements. First with yahoo and then MSN and google. The second site I released content slowly, chose my links carefully and was highly placed in all the search engines in a couple of weeks. Anyway I'm sure this post has helped you immensely! Hopefully someone else will add a more useful opinion.
on a well indexed site if you add links on regular intervel and not let the old links drop your new links will hold more value and serps will improve very quickly like in 5-10 days. The main thing to take into consideration is get 1-2 links every week rather then buying them all at one go. Constant new links to a page with old staying in place will give that page a turbo boost in imo. Moral of the story is make it look natural and try buying blog posts aswell not just those sponsor links or links posted in bunches Wish you luck