Whatever length you want to write your story for, plan your story so it fits into a sensible schedule so you aren't showing four episodes in one season and that's it. Otherwise, people will just forget it's on. It must take an incredibly egotistical TV writer to think that viewers should bother to remember one show or another from one year to the next, just to watch four frakkin' episodes. Maybe if you're The Sopranos, but very few shows are like that.My point was why stretch out 4 hours that you have left to complete your intended story to fit 20 episodes?
You can also look at it the other way around: you're a TV writer and you want to keep getting a paycheck. You can think of a whole lot of premises (that's the easy part! people do it here all the time) Why not choose a premise that has the potential to give you ten years of employment, versus one or two?
Why even bother with one or two year premises? Assuming you're a skilled writer who can create compelling character arcs, you can do a lot more with several years than with just a couple.