Ramping takes days (maybe even over a week). The RF is obviously off most the time though, and the gradient magnets are off too - these tend to be the evil bastards. The slew rate messes with your balance and after a few you come to dislike certain sequences - gradient echoes, diffusion and a few other - loud with high slew rates. You feel like your spinning and this causes slight nausea (in me) and 10 minutes or so of vertigo. Edit for spelling

They're super conductors so shutting them off involves draining all the liquid helium and nitrogen which is very expensive.

Well you can have a superconductor with no current running through it. Just charge the electromagnet with current during the scan, then bleed it off gradually when you're done. No need to turn off the superconductivity to power down the magnetism.

MRI magnets are a pain to setup. It's not an easy task.

I can remember when the NMR (superconducting magnet as well) in our school "quenched". It's a hell of a lot of current to dissipate.

Need to get some tech from Germany to come out and power it back up.

And $75k ish of helium - which is getting pricier by the day, and can at times be impossible to source. And the riskiest time in terms of quenches, is during ramping. Our last quench re-occurred just as they finished ramping it. We were down for 3 weeks ish and the cost excluding helium was huge.