The reason it varies from state to state is because the federal government subcontracts the process out to local state agencies calked Disability Determination Services. There are websites that will give you comparisons of how long different states take.

A lot depends on what you are submitting as evidence that you can't work due to disability. If you have a long paper trail of getting treatment, that helps. If you have failed at a series of jobs, and you can relate those failures to your disability, that helps.