Re: Just how far back does this go?
It is reasonable to assume that it goes back to the earliest SF-86 or similar form completed by any employee in active service at the time eQIP was deployed in 2003. The uncertainty for others would relate to the probable fact that OPM was scanning and possibly doing OCR on a resources-available basis for earlier forms that those no longer in government service submitted. It is not announced whether they worked forward from the oldest paper forms or backward from the newest. Whether a hard limit would exist would depend significantly on the legal retention requirements that govern disposal, something that varies by document type. My recollection is that military payroll records must be retained well over 40 years, partly because of the complexity of military retired pay laws.
For what it is worth, exposure of SF-86 type data for most people would not be nearly as dire as generally portrayed in the press. The document does not contain much financial, medical, or criminal history data unless there is something significant, and most of the questions concern events in the last 7 years, although some of them ask about "EVER". Submission, however, includes a number of releases allowing the government to ask financial and medical institutions and law enforcement agencies for more; however, most who would be eligible for a position of public trust or security clearance would not have a lot that would facilitate blackmail, and those who omitted significant matters from the questionnaire would be relatively unlikely to gain the status requested, although it could happen as various home-grown spies have shown from time to time.
The SF-171 contained basic data similar to the SF-86 but significantly less extensive and could have been used as the starting point for a background investigation. Ordinarily was not, and an applicant for a classified position would be required to complete the more extensive SF-86 or similar document.