Your Social Security benefit amount is based on your earnings over your lifetime. The longer you worked and the higher your average earnings over your lifetime, the higher your monthly benefit will be. Your benefit amount will be negatively impacted if there are years in which you did not work or your earnings were very low. The benefit amount is impacted by your age at the time of application, the amount of your earnings, and the number of years that you worked during your lifetime.
Social Security mails an annual earnings history and benefits summary to workers each year. They also provide an online calculator, which can be used to provide an estimate of monthly disability and retirement benefits. You can find the calculator at the following link: http://www.ssa.gov/retire2/AnypiaApplet.html. You will need your detailed earnings history in order to use this estimated benefits calculator.