The only way to know when is the good time to buy and sell is to monitor the market price value. In that way you will know when to buy, if you need to hold and when you to sell your coins. Just like few months ago market price down but these days its slightly increasing again.
You should invest now because the market is favorable for the new people to invest and get the coins at relatively low price. The current price is not so favorable to buy and the last month was very favorable however you have still the time to invest and wait while the price jumps and you get for what you invest. If you are productive then wait for long term.