When should I get a sight test?
Below are the MINIMUM sight test intervals recommended buy the Association of Optometrists, the department of Health and the Federation of Ophthalmic and Dispensing Opticians.

Patient's Age at Time of Sight Test Minimum Interval Between Sight Tests or Clinical Condition

Under 16 years, in the absence of any binocular vision anomaly   1 year
Under 7 years with binocular vision anomaly
or corrected refractive error
  6 months
7 years and over and under 16with binocular vision anomaly
or rapidly progressing myopia
 6 months
16 years and over and under 70 years   2 years
70 years and over   1 year
40 years and over with family history of glaucoma
or with ocular hypertension and not in a monitoring scheme
  1 year
Diabetic patients   1 year