I've been searching the TechNet archives and the web for a definitive answer to this age-old (pardon the pun) question about how long unpopulated PCBs can be on the shelf before there is concern about their reliability (re solderability, moisture, IMC growth etc etc) and bake out and our extra solderability testing becomes mandatory, for example

Typically, assuming boards are supplied from the manufacturer and stored at the end-user following IPC-1601 guidelines, what are the limits if we assess the surface finish based upon the date/batchcode on the PCB or the pack-date of its packaging and what IPC standard would reflect those timelines?

If a supplier completes an order with over-makes, what "shelf-life" remaining relative to the datecode printed on the PCB is allowable - for my day job we stipulate at least 50% remaining.

Generically, I've always known ENIG is considered good for 12 months, ImmAg  and ImmSn good for 6 months, HASL and "hard gold" at least 12 months or more, etc etc but what are the industry standards that back these "rules of thumb" up for each surface finish?

Thanks (hope all of you and yours are safe and well)

Nigel