The usual answer is once a year or every 10,000 miles, whichever comes first. Short journeys, towing and stop-start city driving all wear a car faster, so high-mileage drivers often add an interim service in between.
Skipping services rarely saves money. Old oil and clogged filters make the engine work harder, and small faults get expensive when they are left alone.
Our servicing page sets out what is included and how long to allow.
Check your handbook too — some newer cars use a variable schedule that the dashboard works out for you.