The most backpay you will get is 12 mos. back from the date of your application from the date of onset (the date they consider you disabled). So if you applied in 2004 and have waited for 4 years for your answer, you are due alot of $$ but if you applied in 2007, say July 2007, you'd get back pay from July 2006 under the same circumstances. It's tricky.
We ended up having them keep my son's backpay because he was also eligible for SSI at the same time, which he received, and once you become eligible for SSDI you have to pay back the $$ because SSDI is considered income. You can't win with those people. So he gets SSDI based on my earnings and SSI only up to the difference of what he was getting on straight SSI! SSDI never changes though and SSI changes all the time based on your income.