When is it Time to Move On?

The Internet Monk has a very honest piece up about ministry and the temptation to quit here. One of the comments makes mention of how we need to surround ministers with encouragement.  That would certainly take the edge off of some of the criticism.  I've seen personal friends in despair during ministry and I don't know if they even wanted encouragement to push on.  Instead they desired permission to give up.  I tend to agree with IMonk that some should not push on but move on and vice versa that sometimes that is the sinful thing to do.  I was encouraged last week by the famous pastor W.A. Criswell that a change of scenery is most definitely NOT the clear solution to a pastor's desire to move on.  Patience may or may not pay off, but we'll certainly never know if we walk away when the opposition gets tough.  When do ministers shake the dust off their boots and when do they dig in for the long haul?  I do not know the answer to the question.