In the modern world, there are two prevailing schools of television production. In most countries, especially in Europe but also frequently in Japan, the aim is to make a short series that only has an initial run of six to ten episodes. If a particular program proves popular (alliteration not intended) it is often renewed for a somewhat longer second series, then maybe a third and so on until the show dies. This tends to result in shows that either disappear before they really capture their audience, like BBC's Life on Mars
, or shows that go on forever, like Absolutely Fabulous
to take another example from the BBC.
The other school of thought, this one primarily practiced in America, is to launch a show with the goal of a seven-season run. This is at once ridiculously ambitious and horribly prone to creativity-squashing. When it works, it's wonderful.