OK, let me clarify that question a bit. I'm not referring to minor vehicle refreshes; I'm referring to redesigns. In other words, a whole new model code. And for the sake of comparison, let's use model years, not production years.
These are the top five that I could come up with, but I'm betting you can do better.
I'm not including commercial/industrial vehicles (vans/trucks), anything Lada, or the original VW Beetle, since those are just too easy.
5. Chevrolet Caprice (third generation) - 13 years; 1977-1990 model years
4. Saab 900 (first generation) - 14 years; 1979-1993 model years
3. Mercedes-Benz SL-Class (R107) - 17 years; 1972-1989 model years
2. Volvo 200 Series - 19 years; 1975-1994 model years
1. Mercedes-Benz G-Class (463) - 23 years so far; 1991 model year-present