The State Department has what I believe are deliberately awful FOIA processing guidelines, which categorize virtually every request as complex, unless all the requested documents are "readily available for release." Exactly what that means, I don't know, but I think it's safe to assume that they would not consider a 30-year-old document readily available.
Nonetheless, the complex vs. standard designation does not, as I understand it, have any effect on the statutory deadline to respond to the request, which is 20 days, regardless of whether it's been designated standard or complex. 5 U.S.C. 552(a)(6)(A)(i).
Of course, all of this could vary somewhat depending on exactly what you've requested. If you've requested a press release from 1992, they should adhere to the 20-day deadline. But if you've requested a compendium of classified diplomatic cables, and they've decided to actually consider declassifying them, that might constitute the kind of "unusual circumstances" where the statute permits more than 20 days. Even then, though, the statute requires them to give you written notice of the circumstances, and the extension usually should not be more than another 10 days.
At this point, you have the option of continuing to wait, calling the FOIA office for some polite sabre-rattling, going through the OGIS dispute-resolution program, or treating the delay as a denial and filing an appeal. You cannot go into court until you have gone through the full administrative appeal process.