We need to accept the facts that it's very difficult now to see a coins doing x10 x20 or even doing 50% returns on investment in this bear period. And nobody knows when the bulls will come. Therefore, it's will be advisable to exist when your coins do 20% returns or even less. I know some investors will not be satisfied with 20% but we need to be objective and understand this bear period will last longer than expect.
I believe you mean to say exit when they're getting around 20% appreciation, which according to me is not a great idea. If you observe past returns given by bitcoins, this is very less and I am not really sure why would people settle for less. As for the bear periods do remember they won't last forever, sooner bulls will be back and one will definitely earn more than 20%.