At some fraction of milliseconds, maybe ? We're talking about pressure difference comparable to big explosion so any break would most likely, on top of near-imm

At some fraction of milliseconds, maybe ? We're talking about pressure difference comparable to big explosion so any break would most likely, on top of near-immediately expanding the hole, compress everything into the sub and possibly heat it enough to combust. 400 atm is only one order of magnitude of a difference from what water cutters use. It's insane amount of force, there is no "small leak" nor "small leak expanding", just "boom"