That's another lifetime-limited thing -- the helium leaks out, and you cannot (for practical purposes) stop it or even meaningfully slow it down. When it's gone, the drives are dead. And the helium leaks by calendar-days, it doesn't matter whether the drive is powered on or off.

Non-helium hard drives are basically limited by their bearing spin hours. If one only spins a few hours a week, it'll probably run for decades. Not so with helium.