Has Star Trek produced the most hours, on screen, of any 'Science Fiction
I knew that it would have to come down to Star Trek and Doctor Who, and at first, I thought that Doctor Who must have more hours of screen time than Star Trek, because the original Who aired almost continuously from 63 to 89 for 27 seasons. It turns out that in the original Doctor Who, episodes were only 25 minutes long, whereas over all of the live action Star Trek TV series, including the original pilot from 1964, episodes have ranged from 43 to 50 minutes in length, and only the animated series episodes were significantly shorter, at about 23 minutes each.
Because Who was off the air from 1989 to 2005, except for the TV movie, and Trek had new episodes airing continuously from 1987 to 2005 over four TV series, it was able to catch up to and surpass Who during this time period, plus all of the movies if you want to count those, since they are 'on screen'. After searching the web, I have found that there has been about 372 hours of screen time produced for Doctor Who over both series, and about 544 hours of screen time produced for Star Trek over its six TV series and 12 films, and 520 if you only count television.
Since the new Who currently has about fourteen 45 minute episodes per season, it will be a long time before it can surpass Star Trek. This is not counting the Doctor Who spin-offs like Torchwood and the Sarah Jane stuff, but I'm not sure that those would put it over the top anyway, or that they should really be counted. So has Star Trek produced more hours on screen, than any other Scifi franchise? I know it has not produced the most of any scripted TV franchise, because you have to count those fifty year old soap operas, but maybe the most of any primetime show.