I was approved with my initial application but they had me see one of their psychologist's and a MD. I had documented my last day at work. My LTD insurance used that date as well. Getting my private insurance didn't help with the SSDI application but it helped me continue to get paid when they tried to cut off those benefits.
I kept thinking I was going to get better so didn't file until over 2 years from that date. They paid me back pay as follows. The first 6 months after my last day they didn't pay as that is the amount of time you don't get paid for. I also didn't get paid for 6 more months as I filed so late. They can only pay back 1 year from when you file. So I got paid back one year from filing and for the 3 months it took for approval of my application.
I also have other health problems that includes mental as well. I was 55 the last time I worked.
The most back benefits you can get is a year from when you filed. Also you can get more for any time you had to appeal your filing. So if you filed and they denied then you appealed and that process took time I think they should pay you for all the time from when you filed to when you were ultimately approved.
That's my understanding of how it works. The back benefits were great as I could afford to replace my washer and dryer and some other bills. I imagine you could use that money as well.