I assume you are talking about Statcounter. kwblue is correct:
If you only install the StatCounter code on one page of your website and your visitors never reload that one page. Then your visit length will always be less than 5 seconds! It is highly recommended to install the same code on all pages of your website you want to track.
I use Mambo so I track all pages that upload in the template. EZ for me. Guess if you had 100’s of seperate pages, this could be a chore. 