Theoretically, zero. However you’re right that the structural demands of maintaining pressure over long term (and, especially, maintaining cryogenic temperatures and high pressures at the same time) are high and there is a large risk of unintended pressure release.
There’s also leakage by diffusion of gasses, which might be non-negligible due to the high pressure gradient, although the diffusion coefficient e.g. of water through steel should be low. Not sure how that works out.
Theoretically, zero. However you’re right that the structural demands of maintaining pressure over long term (and, especially, maintaining cryogenic temperatures and high pressures at the same time) are high and there is a large risk of unintended pressure release.
There’s also leakage by diffusion of gasses, which might be non-negligible due to the high pressure gradient, although the diffusion coefficient e.g. of water through steel should be low. Not sure how that works out.