7 seasons of TNG, 7 seasons of DS9, and 7 seasons of VOY are far from flops. Followed by 4 seasons of ENT, Trek was on continuously for 18 years. Off the top of my head, only Law & Order, the Simpsons, and Gunsmoke were on TV longer than that. Seemed pretty successful to me.
All in the Family has gotta be close (Started in 1971), it spun off Maude (Which spun off Good Times) and The Jeffersons (ended in 1985) which spun off at least 1 short lived Florence show) as well as spinning off/retooling to Archie Bunker's Place and a Gloria Spin off or two.
ETA: OK, checking more thoroughly, I guess it only last from 1971 (All in the Family) to 1985 (The Jeffersons). All the other spinoffs seem to have all finished before The Jeffersons ended. Still 15 years, so, kinda close
Not to mention The Tonight Show
, Letterman's current show, SNL, daytime soap operas and game shows, or Dr. Who
's original run if you're counting British series....