TV drama lengths

I thought some drama serials from Hong Kong, Singapore and Taiwan are long, with some can continue for years at the rate of one episode per week. Only recent years I started watching some drama serial from US. In fact, theirs is even longer, not in terms of how many episodes and how long a drama serial continue, but in the number of years they cover. At about a series of about 10 to 25 episodes per season, 2 seasons per year, a 6 series drama will cover 3 years! Some examples come to mind are the Cosby Show, Friends, CSI, etc. We watch not only the show, and also the growing up of the actors/actresses, especially those younger ones.