For one of my squidoo affliate sales (article's on-page optimizations is goood, has 1500 words) I've started to build backlinks for last 5 days, already 16 do-follow links completed from various relevant sites. Still no improvement in SERP, from you guys experience how long it takes to shows the effect?
There is no way to know. Sometimes your page ranks quick and sometimes it doesn't. In the begining Google is trying to figure out the value of the content on your site so it may jump around for a while. I have had a site on the first page of Google one day and then a week later it is gone. It can go through a "dance" so to speak while Google decides the validity of your site.
Yes as mentioned above. If your keyword has heavy competition, 16 backlinks are not sufficient. Try to increase your link popularity by building more backlinks. FYI: You can also provide your Squidoo URL in the author bio section of Ezine articles. So try promoting the article there.