How long did the most difficult compound to synthesize last?
There is no such thing as the most difficult compound to produce. Instead, there are some hundreds of thousands of "stable" compounds of which we know pretty much everything - melting point, boiling point, heat capacity over a wide range of temperatures, refractive index, speed of sound, magnetic susceptibility, electrical conductivity, crystal structure, and many more. Around that field is a much wider "grey zone" of the not-so-stable compounds (millions of them), which miss some of these properties because they can't be measured.

As we reach out to the far outskirts of that zone, we meet more and more compounds with less and less known properties in each one, because they are increasingly frail and short-lived, to the point that the very words "produce" or "synthesize" (or even "exist", for that matter) do not quite apply to them anymore. Some of them last but a fraction of second; some live longer, but require very peculiar conditions. There is absolutely no meaningful way to arrange them in order of increasing "difficulty".

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