Yeah the instrument panel will tell you when it needs a service.
The Caddy vans at work typically need a service every 14k on the longlife schedule, thats a mix of town use, and ragging it, I believe.
My car was changed to time & distance before I got it, thank god! I once saw a thread where a guy stripped down 2 engines, 1 serviced every 3000 miles, one serviced every 10000 miles. The 3k interval engine was mint, but the 10k engine had a lot of scoring and wearing from the particles in the oil. From that day on I realised the importance of regular oil changes. No car should run oil for more than a year, manufacturers want their engines to blow up, I swear!