i don't hold any of them in first place to want to decide what to do when they get dumped! we all know that sooner or later all the altcoins out there are going to get dumped so the only question is when to get in and how long to stay in.
the answer depends on the market itself and the potential of the ongoing pumps that the specific token has. then as soon as that pump ended you must make your exit instead of waiting around.
Every investor has his own investment strategy that works well for him. You have a point when you say you don't hold any coin for long in the first place because it is obvious you are a short term investor. Every good investor knows that not all coins are worth holding for long term yet others worth the long term hold. This can only be determined through the performance of a thorough research into the project of the coin in order to ascertain the potentials and capabilities of the coin. After this, an investor will have reasons to hold for sometime and never panic sell even when the price drops for a period of time.