Most likely As of live action Spinoffs go, nothing will beat Frasier.
It was so big for so long that I frequently forget it was a spinoff. Then again, I never watched Cheers in the first place. Granted, there is ONE thing which blows Frasier out of the water and it's NCIS. NCIS was a spinoff of JAG and not only has it run 14 seasons so far (to Frasier's 11) but it's spawned TWO spinoffs of its own: NCIS LA and NCIS New Orleans. Granted, Law & Order also had its very successful SVU spinoff.
Simpsons is also kind of a spinoff, and that been going for close to 3 decades