I think the length of the show depends on the material. If you can keep cranking out quality (key word) material, then go as long as you want. But if you're just milking things to stay on the air, then end it.
I agre that:
The way shows are ordered needs to change. Don't give a show an 8 episode order to start the year and then tack on another 10 epsidoes when you find out people like it. You almost assure yourself of unever writing and weird storyline breaks. 12-16 episodes of a serial per season can be plenty to tell a tight, entertaining, engrossing story.
I wish networks would plan shows here and there that have a planned beginning and end. For example, instead of making a pilot or ordering a short season (say, 8 episodes to keep with the example), order two seasons and have the writers carefully plan out an encompassing plot within that time frame. Shoot it, air it, and move onto the next thing.
I understand that can't always work, and networks won't do it, but it would be nice. I mean shit, even if the ratings are low, just pull it off the air and send it to DVD/streaming and let the fans of the show finish it out in other formats. And it doesn't even have to be two seasons since that may be a lot (and expensive). Just carefully create one-season shows...something that has a start, middle, and end and wraps everything up nicely.
Something like The River or Harper's Island. I really liked both of those. I know they weren't too popular, but it was cool to start a show knowing it was basically a long mini-series of sorts and knowing that I would get an end to the story. It sucks so much trying to get into a new show because it could be cancelled at any moment by scared, tepid network execs.