Most adults with no eye problems are advised to have a test every two years. Children, people over 60 and contact lens wearers usually come every year (general guidance; your optometrist may suggest otherwise).
Sight often changes so slowly that you do not notice. The first sign is usually small print held a little further away, or tired eyes at the end of the day.
An eye test is also a health check: the photograph of the back of the eye lets the optometrist compare one visit with the next.