I just saw this question from a friend @saulcolt asking what the lifecycle of a tweet/update on twitter was and it would a great blog post. So with that said my opinion and experience has lead me to believe

That when tweet and update it usually has a life line of 10 to 11 days on search.twitter.com look up your twitter name i personally have 20+ pages of tweets and oldest one was 11 days ago so there ya go

Hope you found this post usefull as this was a spontainous post over my iPhone

Comments and Thoughts are wellcome till next post seeya