It will count how many characters the user wrote so far on Twitter. It isn't perfectly accurate because there is no way to get all the tweets at once.
It reads the last 100 tweets, count their characters and divide it with 100 (or less if there is no 100 tweets) to get the average number of characters in tweets. After it has the avarage number of characters it multiplys it with the number of tweets.
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DalaiLama
Dalai Lama typed ~85544 characters so far.
It is less than the number of characters in:
Latest Tweet: "HHDL will speak at the European Solidarity Rally for Tibet in Vienna, Austria, on May 26th. http://t.co/gwSO5tfo" Sat May 19 09:34:44 +0000 2012 via web