In total for Star Trek TV shows (including TAS) and movies, it's roughly 542 hours (based on minutes per progam).
Gunsmoke, which ran for 20 seasons, 6 as a half hour show (233 episodes) and 14 as an hour show (402 episodes), totals about 449 hours for TV, and that's a SINGLE show, not a franchise. There was also a short-lived spinoff called Dirty Sally which ran for 13 episodes for another 5.2 hours of programming, so make that 454 hours.
I realize the topic is "made for the screen" but I think it's worth pointing out that Gunsmoke started as a half hour radio show, with 422 half hour episodes totaling about 183 hours, which would mean that if you mash up radio and TV you get 637 hours of Gunsmoke alone.
Hell, All In the Family and its spinoffs account for 816 episodes and 345 hours.
To be fair, I did specify 'scifi' franchise
Gunsmoke was a western. As for radio, Doctor Who has a bazilian audio dramas, but those aren't 'screen' Still, they are produced stories. Although, I think Star Trek probably has more original novels than Doctor Who, but since you can't count running time on novels, it's hard to factor those in.