Depends on the organ. Hearts can last just a few hours usually less than 4 is ideal. Pancreas transplants should be done within 12. Livers within 24 and kidneys can last up to 48. Now that said most centers transplant them much faster than that. The faster an organ is transplanted the better it works. Organs that have been on ice for a prolonged time have a higher rate of not working or not working well.
Seconds. Organ donation must take place seconds after cardiac death. In general, donors with brain death are taken to the or and have mechanical support removed then the donation takes place. If a patient suffers cardiac death before the or, they aren't a candidate for organ donation, but may qualify for tissue and cornea donation. Please speak to your local donor network for more specifics.