Tweet now - speak later

How long does a Tweet last?  Minutes, hours, days?  Based on my own traffic flow to my site from tweets it is not very long.  So how often should you RT if indeed you should.  I think that repeating the same thing over and over can become somewhat boring but is this not what marketers do?

Someone more qualified than me wrote an interesting post on the length of their Tweets and RTing: The life and times of a Twitter link.