Cool, dry, dark and sealed, with humidity controlled rather than merely monitored. Dried herbs are hygroscopic, so they equilibrate with the air around them, and a warm humid warehouse will undo a good dry within months.
Why it works this way
Three mechanisms degrade a stored herb, and all three are slow enough to be invisible until a customer complains.
Moisture uptake raises water activity toward the level at which moulds can grow, and it also promotes clumping in cut material. This is why the liner and the seal are quality-critical: a polyethylene liner properly closed is doing more work than the carton around it.
Oxidation removes volatile oil continuously. It is accelerated by heat and by headspace, which is the argument for vacuum packing premium grades and for smaller pack sizes where a unit will be opened and used over weeks.
Light bleaches colour. Green leaf material is the most exposed, and because colour is often the visible grade criterion, fading is a commercial problem rather than a cosmetic one. A lit warehouse with clear film on the pallets will fade a lot that would have held in the dark.
What follows in practice
Specify a maximum storage temperature and a maximum relative humidity, not just “cool and dry”. Use an opaque or foil-lined outer where colour is the grade. Keep herbs away from strong-smelling goods, because dried leaf takes up odours readily. Include a humidity indicator card in the pack where the chain is long, and log warehouse conditions rather than assuming them.
Then state the shelf life together with the storage conditions it assumes. A shelf life without conditions is an argument waiting to happen.