when you are getting close to the ATR from the move from the recent low.
eg, if yesterdays low was on the dow, say 10,800, and the atr is approx 75-80 points, i would start to think about an exit , assuming an breakout long at the 10,870-10800. and then look for confirmation of a reversal. eg engufling bars, 1-2-3 patterns etc.
remember, its best to let your profits run as long as possible,..
fc