Right, the only time you have to replace it is when you service the system and it is open. The drier is the very last componant that you would replace in the system and immediatley vacumme the air and moisture out, then the drier takes care of any residual moisture that would contaminate the oil. If you dont replace it if the system is open your system will fail, if you unseal the drier, let it sit around your system will fail.
So to answer your question you do not need to periodicly replace it unless your system has a leak and ran out of charge.
95 E34 530I V2.37
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