At 125 neuro patients per m3, we're talking room for 25 million patients in a single 200,000 m3 tank. Total boiloff would be 40 m3 per day, which would take 5000 days if the tank is full of cryogen, or 1250 days (~4y) if the patients take up 75% of the volume.

If we wanted to get it to the century range, I wonder how much thicker the insulation needs to be... a factor of 25? Number of watts would need to go down to 200 milliwatt/sqm.