I usually hold, accumulating when they are looking cheap as long as the stock does not already constitute a disproportionate part of my portfolio. If, however, I judge that something has happened to make the no longer the "story" it was when I bought it (management blunder that will take years to recover from, disruptive technology, adverse legislation, long-term change in the macro environment etc), then I get out, generally after a period of costly dithering.
