If they go another season, after killing jack, and after every major character has left the show that was with it the first season, it's time to let it die.
The Xfiles serves as a great example of what happens when a good show goes too long, especially under FOX. After the 5th season or so, it was dead. 2 new characters that you couldn't give a damn about, one old one that barely did anything and a whole lot of self-parody. I can't imagine 24 being good w/out any original cast.
As for this season, its getting better. The first 2 episodes sorta made me feel "Eh" about it, but now that the plot thickens (yay cliches!) i'm more inclined to watch as soon as it's done taping (comercials suck!).