If your doctor determines you are unable to work for 12 months or more, you should file for benefits immediately. By waiting longer than necessary to apply, you risk losing benefits to which you are legally entitled.
If you are eligible to receive SSDI benefits, you can only recover retroactive benefits for the twelve-month period preceding the date you submit your application for benefits. If you wait more than a year from the date you stop working to apply for benefits, you risk losing more benefits with each passing month.