For what length of time can your heart be stopped before you can successfully perform cpr?

Depend: Depend in few factors. Like age, co-morbidities( prior medical condition affecting cardiopulmonary reserve), lifestyle( obesity, smoking, too much alcohol, nutrition status), the situation ( increase survival in cold water drowning). There was a case of a 54 y/o with cardiac arrest for 90 min who survive. Dr roger d white directed the cpr. But, early the better. CPR is a very good skill to have.