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depends on a number of factors. But if the coffee has been dried and stored well, then 9-12 months from picking should be possible. old coffee exhibits what we call fade in the green coffee business (i work qc and logistics for a small import/exporter) and the characteristic taste is a lack of sweetness with a papery/wheaty flavout.
if the coffee is roasted dark, this will make no difference. you can also hide coffee in blends.
How long does it take for the green beans to go (noticeably) stale? Would an average consumer be able to tell the difference between a new crop and the previous year?
I suppose with the spread of coffee growing regions, you've got new harvests coming on stream throughout most of the year?