It depends on how yourself, but that's not the problem because ive passed it before. If i predict in long-term it will fall foreverr so i will sell soon then try to take profit with another coins.
In fact it is depending on your trading strategies, if you are a short term investor then no doubt that it is time to sell, because you have to change your planning and buy again in low price, but if you are a long term investor then 15% is not such a big drop you need to wait for the market recovery.