hi friends i read about using long tail keyword i want to know the limit of long tail keyword and how i found them because Google trend z doesn't show long tail keyword so please help me
The limit of long tails is usually with the traffic, long tails get less traffic. Long tails are just keyword phrases that are 2 + words long, longer if your market is very competative. I use Market Samurai for my keyword research, its very good, gives you good information about the competition too.
There might be no limit for the long tail keyword if you really know that this phrase is being typed constantly. Well, Google External Keyword Tool won't give you long tail keywords, but you can use some keyword tools from stand along developers (if they can provide you with long tail keywords ) For example you can read this article, because to my mind they give their users such keywords: http://xedant.com/help/en1.php