Helium Drives - Long-term use

Nowadays, the bigger harddrives are often filled with Helium instead of air. However, Helium is a very volatile gas and I have problems in believing that the Helium stays inside the drive chassis forever (also the S.M.A.R.T. ID 22 "Helium Level" supports my fear).

Does anyone know for which duration those drives usually keep their Helium before it is gone by leakage?
What will happen to the drive after the Helium is gone (and is replaced by air)? Will the drive fail?
Does someone here have...

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