Depends why you entered. If you are entering because you expect to catch the next leg of upward momentum, then sell once the momentum is no longer there. You may well find that 50% of the time (or more), the momentum resumes later. That doesn't mean it was a mistake to sell when the initial momentum ended.
If you are entering to catch a major trend, then you should not sell until there are clear signs the trend is at an end. It is pretty much impossible to ride a multi month or multi-year trend without experiencing some pullbacks. But it IS possible to ride a stock up 200%+, or ride a futures market up 100%+, using trend-following techniques with a wide trailing stop.