Doctor Who is
I think still the longest continuously running sci-fi series, and it can be proud of that. But in terms of screen time it is curtailed by its very nature.
There are a number of factors why Doctor Who lasted the distance though. The ability to reboot the entire cast/format every few years being the primary one. The Star Trek format/s tended to be set in stone to a certain degree (a cast member here or there could be changed out or moved around, but it wasn't by any means a cast clear-out, and the core cast of each series remained the same for their entire run). Doctor Who could literally shake up everything
and still be a viable proposition. It was part of the format, and part of it's longevity.
After seven seasons, even Tom Baker was beginning to look old hat. So the ability to just change the entire cast for some new guys and revitalize the entire format as a result was a key component. In a lot of ways that resembles soap opera production more than anything else; the format and the characters have each got a degree of seperation that means the former is never affected by the departure of any of the latter.