It's very simple, you can sell when you think you are on profits enough or when you feels you don't have anything to lose from the market, otherwise I wouldn't encourage you to hold altcoin for long time than short time and, altcoin is always good when you see profit and you take it quickly without having to wait for it to double extremely. When you hold altcoin for long the chances of you losing your investment is very high because whatever that happens to Bitcoin automatically it gives weakness to the market causing serious panicking among traders to dump the coins without any noticed.