I have some experience with NMR from a chemistry perspective. The magnetic field is generated through a super conductive coil. Cool it down with liquid nitrogen, generate the current and you have a superconducting magnetic field as long as you keep it cold.

If it warms up, it’s no longer superconducting and all that current starts generating heat which boils the liquid nitrogen off, which can be quite spectacular. Plenty of videos on YouTube.

So yes, turning the magnet on and off is not trivial at all, but as long as you keep it cold, you’re good.