You can hold coins for long term if they have good team behind it and anything that is 6+ months old and still trading on big exchange. Doge, DRK, LTC, BTC, BC, VTC are some good ones.
If it's new coin I try not to hold it for longer than a month.
I agree! 80% of new coins launched are gone in the first 30 days, so every new coin is a huge gamble. If you have made money off of this newly launched coin, you could sell whatever amount is needed to break even or to give yourself some profit and then leave the rest in it. Or, you could sell it all, and buy back in (if it is one of the 20% that may make it past 30 days) when the price comes down after the initial rise and / or it has at least 6 months of good history.
DNotes was launched in February and is positioning itself as a long term investment. It has had a steady appreciation in value since then and is worth checking out if you are looking long term. I've never owned Darkcoin or even looked into it, but it was one of the few that was profitable on a chart I did at the end of December. The chart was from March 8 (the day DNotes made it onto CoinMarketCap) to Dec 31, so Blackcoin was also profitable. Blackcoin was hurt by that huge runaway price spike and it will take a while for them to rebuild confidence, but it does look like they will stick around. I haven't checked out Vertcoin in a long time, but I did buy and sell some quite a while ago. BTC - everyone should have a little.
Try to spread your investment out so you have no more than 20% in one currency, at least to start.
Good Luck!