I have been bulding backlink for 15 days I checked google webmaster tools if it appears however the backlinks aren't appeared in webmaster tools I wonder when will it be counted ?
Actually it depends on the quality and accuracy. Though it can't be explained surely, because google will not tell you about this and this is like mystery. But normally it takes few days to show but 15 days is too long. You should check everything. May be there are some problems.
the links you are adding must be dofollow in order to count as a backlink also, their is no such promised time limit and sometimes it may even takes months if some trusted and quality site will link back to you then it will be counted fast & vice versa
I don't think there's a straight answer to your question, infact there's no answer to any question that involves the term 'google algo' and I am afraid this does!
when google crawl your text link (anchor link) which you put in others site & it is relevant to your website then google count it as a backlinks.
it always take time, keep updating you site and when spiders crawl you site again, you will notice some backlinks will appear
i suggest that you just keep building backlinks and ensure they are high quality. trying to bend your mind over when the Big G will update your links will drive you mad. It is typically right before a PR update. every 3 months or so.
Improve the quality of your site to increase crawl frequency. its not a wise idea to look for backlinks to show up within 15 days I will give it at least a month.
You cant ever know when google will crawl and eventually index your backlinks you can just hope it does But pinging your backlinks is a good way to get them crawled by google
I agree with EatGreedyTx, you will not see any of your backlinks until Google crawls them. That might be a matter of days or weeks. It just depends on how often the sites you are using to build the links get crawled.
I also agree that there's no specific time. It really depends on where the links came from. If a link comes from a quality site and well-visited by search engines, then it will be crawled much faster.
Google webmaster tools provides links to your site but there are some loop holes over there too. Google indexes links as fastest as it is possible but same time it is not necessary that as soon as the link get indexed it will show the backlink data on your webmaster tools. It will certainly takes some time. Wait for 10-15 days after the page containing your link get cached. And if it not cached then start link wheeling to make those link noticiable.