The substantial gainful activity (SGA) amount for 2010 is $1,000 per month. You can see the SGA amounts since 1975 here.
The SGA determination is step 1 of Social Security's 5-step sequential evaluation. For Social Security disability or SSI, you must have (or expect to have) a 12-month period during which you do not have earnings at the SGA level.
If you are an employee earning more than $1,000 per month, you are probably not eligible for Social Security disability benefits. See 20 C.F.R. 404.1574. I say probably not, because other factors are considered, including IRWEs and UWAs.
If you are self-employed, the test for SGA is a more complicated. See 20 C.F.R. 404.1575.