Generally the large producers give 2 years to Darks, 1 year to milk and white. Also note best before date and expiry dates are two different animals.
With chocolate, use the best before, as it does not really expire for quite a long time after the BBD.
Free water (also known as water activity) can be a good proxy for longevity. The more water, the shorter the shelf life. There are some (comparatively) inexpensive water activity meters that will let you know precisely what the water activity of any particular batch is. "Raw" sugar often has, in my experience, much more water in it than refined sugar.
I don't remember, off the top of my head where you want the water activity to be. I'll get in touch with Sebastian and ask him to voice his opinion on this topic.
It's also fair to say that storage conditions will affect shelf life - generally too hot is worse than too cold except for the issue of moisture and condensation.
It's also helpful to remember that chocolate never stops crystallizing and much dark chocolate will eventually become brittle in texture (form VI crystals predominate). This can happen well within two years, for many reasons. Melting the chocolate down, keeping at 115F (~45C) for at least an hour with agitation to melt out all the form VI crystals then temper and the chocolate (as long as there is no other defect that affects taste) should be good to go for some applications.
Thank you very much for the information. very interesting the information about the water activity; Yes, the raw sugar has more water... and the storage conditions are also important.