How many years does a sf86 application investigate?

It depends on what level of clearance you are applying for. As of late 2007: Top secret SSBI requires you to go back 10years NACL or NAC requires 7 years. Recently, even though you may only be applying for NACL/NAC, they will require information back 10years anyways. You can safely assume they will investigate that far back, if not further. Visit/contact http://www.omp.gov for up to date/detailed information.