After playing healers in several games, I take into account several things. Firstly, any serious injury is not going to be healed immediately without some repercussion to either the healer or the patient. It can vary from exhaustion on the part of the healer, which for large amounts of injuries can quickly incapacitate an unwary healer, to ravenous appetite and taking things easy for the patient.
I also take into account the scene and the person being healed's roleplay. After all, a debilitating injury that is part of a storyline and needs a certain cure should not be healed with a regular application of magic, where as a wound just got for the sake of flavour can be healed quickly and with a little more effort, not leave a scar.
It comes down to communication between the person being healed and the healer. Most healers I know are very careful to give the patient exactly what they want.. most of the time.
I have only ever put someone in a medically induced coma once however, as that is taking their character out of play and no fun, but that was cleared with the PoPatient first.