How long after someone stops breathing can you still revive him?

Depends on temp. Depending on the patient's temperature from 4 minutes to hours. People who are immersed in very cold water may often do quite well up to 1-2 hours after drowning. Most patients suffer severe brain injury after 2 to 4 minutes of no breathing.
NO ABSOLUTE TIME CAP. In other words, depending upon the condition of the patient, the cause of the problem, the "time down", the end organ damage and the effectiveness and conditions of the resuscitation, full recovery may be possible after minutes to hours. However, as a rule, after cardiac arrest, outcomes are not good, unless effective resuscitation is accomplished rapidly ... Within minutes.
5. Whether or not a patient can be revived depends on many factors. In general, younger patients have a better chance of revival than older patients with pre-existing vascular problems or cerebral vascular disease. Additionally, the circumstances of resuscitation are important. For example, patients that fall in cold water may be revived after many hours if their body temperature cools rapidly.