Can the brain regenrate after cardiac arrest after 30min.?

Generally not: It depends on the specific situation, but if the brain has been deprived of oxygen for more than 12 to 14 minutes, the damage is (usually) at least partly irreversible. A relatively new technique called therapeutic hypothermia improves the statistics somewhat, but the outcomes for many patients are still poor.

Depends: It depends on several factors, not the least of which is body temperature.