There is no standard length that dictates when a search term is considered a long-tail keyphrase. Most definitions start considering a phrase long-tail at three words or more. However, the length of a phrase does not tell the whole story.
What sets a long-tail keyphrase apart, in addition to number of words, is its specificity. Long-tail keyphrases typically target narrow questions or precise definitions.
The sheer beauty of long tail keywords for a law firm can be summed up in one sentence: “Low volume traffic that converts is more effective than high volume traffic does not convert.”