You guys are forgetting the 1994-96 Impala SS.
Also don't forget the trucks: 1994-96 S-10 SS, the 2006-up Trailblazer SS, the 2003 Silverado SS, and the 2003-06 SSR.
S-10 SS:
Trailblazer SS:
Silverado SS
SSR:
If you mean continuous:
Impala = 1961-69, 1994-96 (9 years + 3 years = 12 years total)
Nova = 1963-76 (replaced in 1977 with the Rally) (14 years)
Chevelle = 1965-73 (yes, there was a 1973 SS Chevelle, replaced in 1974 with the S3) (9 years)
Camaro = 1967-72 (replaced in 1973 with the LT), 1996-2002 (6 years + 7 years = 13 years total)
El Camino = 1968-87 (20 years)
Monte Carlo = 1970-71, 1983-88 (2 years + 6 years = 8 years total)
So the El Camino is the longest continuous (20 years) with the Nova second (14 years).