If anyone is curious what pushing the button to turn off (AKA "quench) the magnet looks like, there's this video of an MRI machine being decommissioned:
You push the button, then 15 seconds later the liquid helium is vented through a pipe on the roof of the hospital (it's quite a spectacular display), and then the superconductor starts to heat up and no longer be a superconductor so the current that's been flowing through the coils (they are energized once, when the machine is first installed, and then continue flowing forever so long as the superconductor is superconducting since there's no resistance) and the magnetic field decays to nothing.
https://www.youtube.com/watch?v=9SOUJP5dFEg
You push the button, then 15 seconds later the liquid helium is vented through a pipe on the roof of the hospital (it's quite a spectacular display), and then the superconductor starts to heat up and no longer be a superconductor so the current that's been flowing through the coils (they are energized once, when the machine is first installed, and then continue flowing forever so long as the superconductor is superconducting since there's no resistance) and the magnetic field decays to nothing.
It's not an instantaneous process.