The long-term prognosis for coma depends upon the cause and length of the coma. Recovery from drug overdose coma depends on how long the person stopped breathing. With a diabetic coma, recovery is determined by how long the brain was deprived of sugar. With a coma brought on by heart attack or loss of oxygen to the brain, chance of recovering is slim, especially if there is no responsiveness within a few days. A coma usually lasts from a few weeks to a month, and without complications, a person can expect to gradually gain full recovery of physical, intellectual and emotional health. The longer a person is in a comatose state, the less likely full recovery is possible.