As soon as possible! You should file soon after you become disabled and are unable to work. You do not need to wait twelve months to apply. An application for SSD may also be filed after the death of a disabled worker. It must be filed within three months of the death of the worker.
When you apply you should ask to be considered for both SSD and SSI in case you haven’t worked long enough to be eligible for SSD. Even if you are eligible for SSD, if your earnings are very low, you still might benefit by applying for both.

The whole process can be very time-consuming. The sooner you get started, the sooner you may begin to receive financial assistance from Social Security.