This is difficult to answer without defining "relapse" and "crash" very specifically.
If relapse is a return to a level of illness which is generally incapacitating, but not relentless unbroken misery, following a period of time where one was in remission -loosely defined as being well more often than one is ill- then my answer would be >10 years. But, if relapse is defined as a state of relentless, unbroken misery during which one cannot for a moment forget one's symptoms then it would be much shorter.