My experience with the same set up as yours Vitt was that my chains lasted 12,000 to 15,000 miles (about 19000km to 24,000km) on the average...EXCEPT when I used wax based lubes and the chains average life fell to 5,000 miles (about 8,000km). I also keep my chains really clean so that may play some role in that chain life. Newer bike now use thinner chains so the component manufactures can squeeze more and thinner gears into the same basic space as your old 6 speed; those thinner chains (and gears) do not last as long as the older wider chains. The average life of one of those thinner chains is about 3,000 to 5,000 miles. Welcome to the modern age.

Your shorter chain life may be due to two reasons: 1) your not cleaning the chain and gears as much as you should; and/or 2) your using wax based lubes.