I've come to see depression partially as a semi-stable brain-state with reinforcing feedback. Some of that feedback may be chemical while some is psychological (thought or feeling). It may even persist once the major stimuli that got to that state have been removed. As such, any intervention that pops the patient out of that state could be permanent so long as the other things that led there don't happen again. Or if those things are still present, any intervention will be short lived.