Personally, my strategy is "holding" when it comes to cryptocurrencies because I bet on the long term of this emerging technology. I do think the market will get way bigger by 2020. I made most of my benefits by holding altcoins on Poloniex, earning money with the automatic sell option. So the good time to cash out your cryptos would be when you need some extra cash, since I think you should only invest money you can lose. Otherwise just hold it till you consider you have made enough benefits. The thing is to have no regrets if you cash out too early!