How long can a person be in a coma before they cannot recover?

Unknown. All depends on what caused coma and type, extent of brain injury.
Depends on the cause. There is no set time, but recovery ultimately depends on the cause of the coma. For example, if the coma is caused by severe illness in another part of the body then when the body recovers the person will often improve. If the coma is caused by damage to the brain there may or may not be recovery depending on the part of the brain that has been injured and the extent of the injury.
Highly variable. Some pts have awakened after months, and even years, but this is quite rare, and is mostly seen in younger patients. This raises the question of how we might handle coma, and each case is very different. The american academy of neurology has issued some position statements on these types of cases. Www. Aan. Com.