I'm getting about 6 months/3500 miles to a chain on my road bike. I am not fastidious about keeping it clean and shiny, but I do lube and wipe it down regularly (monthly or about 500-600 miles), or when I ride in rain (rarely).

I replaced my cassette most recently after about 15,000 miles. No skipping, but just seemed like it was time. Shifting was a bit crisper once I changed it.