The Social Security Administration calculates the duration you have worked in terms of quarters of coverage. For every quarter that you are employed in a position in which you pay Social Security benefits you get one credit. These credits count toward benefit coverage under Social Security. In a sense, Social Security is akin to your health insurance coverage; for every premium you pay you're entitled to coverage for the period that the premium covers. Gaps in covered employment, similar to gaps in paid premiums will result in less coverage.
Quarters of coverage are also factored against your age at the time you attempt to claim benefits. The younger you are the fewer quarters of coverage you need. Before age 24 you need 6 credits in a three-year period to qualify for disability coverage under social security. If you are between 24 and 31 you need to have credit for working half of the time between age 21 and the onset of your disability. If you are over age 31 you must have 20 credits within the 10 years before the onset of your disability.
Individuals who are interested in finding out if they qualify for social security disability before they file their claim can visit the social security administration website and view the table of credit requirements, which shows how quarters of coverage are calculated and how they impact your eligibility for claims. Individuals who are blind or become blind may be able to circumvent their credit requirements due to obvious need.