You will only have that thought of exiting the market whenever you go through some negative experience, because if you do get money from your investments then there is no point to stop doing it.
To me i think that it is difficult to know the perfect time to exit from trading, but still when a person realize that he has got a good profit from his investment then he should exit from the market, i think that he should also get opinion and prediction from the experts and then should act according to their prediction, i am sure that it will give them a very good profit.