Optometrists want to see you every one to two years.
VISION If you are young and your vision is good, an exam every two to three years may be fine. As we age, an optometrist may wish to see us every two years — or more often if over 40 or diagnosed with diseases that require monitoring.
A typical exam includes an ocular pressure test for glaucoma. There may also be peripheral vision tests and an interocular examination. A Snellen  eye chart (first designed by a Dutch ophthalmologist, Herman Snellen in 1860s) is used to determine visual acuity from standard distances. An eye care professional may have several on the wall at a fixed distance or employ a random eye chart generator. The purpose is so that patients do not memorize the text and skew test results.
If a person cannot achieve a visual acuity of 20/200 or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. This means that a legally blind individual would have to stand 20 feet (6.1 m) from an object to see it — with corrective lenses — with the same degree of clarity as a normally sighted person could from 200 feet (61 m). A person with a visual field narrower than 20 degrees (the norm being 180 degrees) also meets the definition of legally blind. 
Download Eye Charts
ClinicalPosters.com offers a Snellen eye chart like the one pictured on this page as well as link to a random eye chart generator. The latter, written by Alejandro Saksida, can be viewed within a Web browser window with optional print function. For the illiterate, pictorial graphics can be substituted for text; The Ultimate Random Snellen Eye Chart requires a Macromedia Flash plugin.
These charts can benefit non-professionals who wish to see if their current eyeglass prescription is weakening or verify whether a new prescription has improved vision. If taking a test in front of a computer screen while using the random chart generator, results may vary depending upon screen size and resolution. Versions are available for different monitor resolutions. Click "More Information" on the product page to view the instructions and read-me file. Eye care professionals may adopt charts as they see fit. Only a medical professional can provide prescriptions.