I don't know why all say 3 -4 weeks, I have heard it all the way from the so called self named Guro's here on BHW and all the noobs.
When I do SEO and I have something that works I can see immediantly impacts on my ranking.. it don't take a day or 2 or 3, it works almost in real time. If I built shit I can wait weeks and maybe some ranking inprovement happen or maybe not.
When I buy links from all these Masters in SEO it can take 3 - 4 or 8 weeks before I see anything because it is just crap in a box they sell.
I'm very aware of that it from time to time can be delayed because the links you built need to be indexed, but it all comes down to quality over quantity, high quality (and I mean really high quality) get indexed almost right away and all other stuff get indexed as per the quality of that link.
Good luck.