Personally, I hold my altcoins for a couple of month while I see how the project is progressing. If it is progressing well and people are using it and so the value of the altcoins increases, I will continue holding them. If however I see that the project is not progressing and even after 6 months the price is dropping, I will sell.
I think people who sell their altcoins soon are either worried about the altcoin dropping or need the money at that point.