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For me, there are two rules of thumb here.
After 48 hours you should see some responsiveness.
If there is going to be significant recovery, give it at least a week.
Hypoxic brain damage is tricky, and I have seen the plug pulled after 2 days, which is too soon.
Yes the odds are poor, but in my experience, not that poor.
OK, so that is an initial answer….
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