These cars (provided they have no major rust problems) will last about as long as you are willing to keep them up. I have seen early 80s 380 and 500SECs with close to 300,000miles (500,000km) on. Once you have the car paid off, even spending 3 or 4 thousand a year on a car like that to keep it in top shape becomes cheaper than buying a new car. The car will last as long as you want it to.
1995 E420 - retired
1986 420SEL - retired