Third Watch ended on May 6th, 2005. The show ran 6 seasons with a total of 132 episodes. Originally, it was slated to run only 2 seasons. Due to high fan demand and large advertising revenue, the show continued 4 years beyond its original length. It's impossible to draw a relevant parallel for Third Watch's run (beyond Lost which was only written to be one season long).
The final Third Watch Episode ("Leaving Camelot") was written mid 6th season. The show lost endorsement from NBC after its popularity slipped in the last season. The story had gained a rolling storyline that could have continued for an indefinite period, but without advertising support, even the best show will fall out of prime-time's favor. The final episode has been called "rushed" and "anti-climatic" as it is lackluster when compared with the average quality of Third Watch.
When the Third Watch pilot ("Welcome to Camelot") was received so warmly that advertising support for the program grew tenfold between the pilot and the second episode ("Anywhere but Here"). Fans had no trouble relating to the "everyman" quality of the characters. At the peak of its popularity, Third Watch had an average 75 Million viewers, with advertising sponsors like Pepsi, Toyota, and even Paramount Studios. In spite of the show's great popularity, writers struggled to keep stories (that integrated all three main elements of the plot) flowing. Some episodes are highly derivative of other popular television shows from the same period. Several episodes feature story lines taker directly from the shield. First Watch wasn't a good show because of original plots; it was a great show because of amazing presentation.
The only television show to run more than twice its original life span (other than Third Watch) was Lost. Lost is still running today, and some argue that the storyline has become totally nonsensical. Third Watch stayed true to its original
concept throughout its 6 season life. It had main characters come and go, but the main operating scope of the storyline stayed on point despite changing times.