It's really a case by case basis. As others have said high risk and high reward. One way to do it is to spread your investment around so it's not all in one basket.
this is not true for crypto market. Let say you invest in DGB, STR, XRP, SYS... on June now your fund is arround 30%. Sometime choose right coin is better.
I'd say the majority of projects aren't worth holding for the long term.
Unless they have huge communities or great technology.