My retroactive backpay took quite a lot longer than other people's, long enough that I had to call in, only to get a generic response. In total, I think my wait was around 3, pushing 4, weeks.
I suspect that because mine was a more complex case of adding dependents that otherwise had no SSNs at the time of the claim, to backpay for them too, and the fact that it was a nearly a 15 year long appeals process ending at the VLJ. As such, there probably needed to be an audit of the amount you can only guess showed up in my bank account before making the disbursement.