Hi, LAgalAS. I'll quote from SSA's web site:
During a trial work period, a beneficiary receiving Social Security disability benefits may test his or her ability to work and still be considered disabled. We do not consider services performed during the trial work period as showing that the disability has ended until services have been performed in at least 9 months (not necessarily consecutive) in a rolling 60-month period. In 2006, any month in which earnings exceed $620 is considered a month of services for an individual's trial work period. In 2007, this monthly amount increases to $640.
Here's the link to the page I quoted: http://www.ssa.gov/OACT/COLA/twp.html.
Let me know if you have further questions.