In an instant unless OceanGate's patented monitoring system actually worked. If it did work, they would've had at least a couple of seconds to panic.

>Rush planned to pilot the sub himself, which critics said was an unnecessary risk: Under pressure, the experimental carbon fiber hull might, in the jargon of the sub world, “collapse catastrophically.” So OceanGate developed a new acoustic monitoring system, which can detect “crackling,” or, as Rush puts it, “the sound of micro-buckling way before it fails.”

https://worldwide.espacenet.com/patent/search/family/0776658...

I don't suppose there is a huge line to license this technology.

In an interview with James Cameron, he mentions[1] that he and other submersible industry experts believe that they did in fact have warning, as they had dropped the weights (which were found far away from the debris field) and were attempting to resurface before the implosion:

[1]: "This Oceangate sub had sensors on the inside of the hull to give them a warning when it was starting to crack [...] they probably had warning that their hull was starting to delaminate. [...] It's our belief - as we understand from inside the community - that they had dropped their ascent weights and they were coming up trying to manage an emergency." https://youtu.be/rThZLhNF_xg?t=472

Or succumbing to paranoia that others might murder you to survive themselves.