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.
Well, Doctor Who has not run continuously
from its debut, since it was off the air for 16 years between 1989 and 2005, except for the one-off TV movie. Still, I think that original 27 season run may still be the longest running, or at least I can't think of any other scifi show that matches it.