It runs for 36 months whether you work or not. During these 36 months, he/she is not due a SSDI check in any month(s) substantial gainful activity (SGA) is above the 2007 limit of $900/month. However, in any month(s) gross earnings are below SGA, his/her check is due. Basically, this means that during this 36 month period, “disability eligibility” is protected. Social Security will presume an individual still has a disability during all 36 months, even if gross earnings are over SGA ($900 / month in 2007) in some months. Because of this “presumption of disability”, an individual will not have to reapply to receive SSDI when earnings fall below SGA, although he/she sill have to notify SSA and provide pay stubs.