The easiest thing to do is to rethink if the fundamentals of your coins are still okay, if they are the easiest thing is to do nothing (and 80% times the best you can do), instead if the fundamentals are broken or have changed usually the best you can do is buy bitcoin and/or ethereum.
Whenever your altcoins are dumping for longer time that usual, it is clear signal of their complete disappearance from the market and devaluation forever. Under such circumstances, get rid of these shitty coins as soon as you can. This is the sole reason why I dont like getting involved with altcoins. They all are very risky including eth. I am not holding any altcoin for more than 2 weeks.
Times I think of dumping it but still not doing it since It will be based on our knowledge to choose alts to hold too. Holding alts that are atill in development status that are really interesting can pump after development. Though we can't rely deny that there are tons of shitcoins now.