well it all depends on what the setup of the shows. like for example star gate, becaus they use a worm hole that takes them from planet to planet and the combinations are limit less so is the show. but if the show is limited to one area or set idea then the plain of options is limited.
space travel ones are limited e.g. star trek or to a lesser extent gate(i guess it shouldnt but they all seem the same to me) but Doctor Who is unlimited (if Dr Who episode start getting repetitive i'm gonna storm BBC and give them a taste of creativity, MAC-10 style, unless they contain Daleks, can never have enough Daleks )
Depending on the setup of the show, I'd say 2-3, or unlimited. Of course, if they go into filler, even if it isn't repetitive, it sucks.
Depends upon the show of course. Shows like Battlestar Galactica or Babylon 5 which have strong main plotlines should end when those plotlines are resolved. E.g. Battlestar Galactica centers around finding Earth, whatever happens on Earth once the find it doesn't change the fact that they have no where else to go. Shows that center around exploration/discovery, eg Star Trek, Stargate, Doctor Who can last a lot longer because when one plot line finishes they can always discover others somewhere else. E.g. Stargate had the Gua'old and once they were finished with them they discovered a new enemy, the Ori.
