Setting up your goal price and if your goal price will reach then that is the best time to sell your altcoin. Most of us prefer for long-term because of less stress and less risky.
Don't hesitate to sell out when it already reaches your target goals, the coins always have a sway and if you sell it out you can always go back and do the same thing, better to gain experience and explore every aspect of profitable strategy, combining short and long term is also good for experience traders, as they are not against the time each time they've trade.