It's never become the longest running prime time series, nor will it ever earn that accolade.
The Simpsons is the longest running American animated program & the longest running prime time scripted comedy & the longest running prime time American sitcom but the title of simply 'the longest running prime time series' belongs to The Hallmark Hall of Fame which has been airing for 64 seasons [& still does] followed by Walt Disney Presents which has been airing for 54 seasons [& still does].
If you want to narrow it down to weekly series, 60 minutes is the longest running prime time series debuting in 1968.
Is it the Simpsons
The Simpsons would become a 'classic' cartoon by virtue of it's longevity of 20 + years .
By the creating the Simpsons.
Season 11's E-I-E-I-Doh
it debuted on Fox in 1989 & in Canada in December, 1990.
its Treehouse of Horror XVIII its a aswome episode
probably by running
Daily Syndication [1993] Cable Syndication [2014]
Africa
a surgeon
No such episode exists however, In Bart to the Future, it was revealed he was the president before Lisa.
You will become fit and healthy.