In my personal experience with altcoins so far, though not yet fully experienced, the best time to buy is whne they fully in red and when in seems like nothing with help to recover them, thats the time to buy. Wait for the dip and buy, our mistake is we tend to be greedy and wait to see some gain and we get tempted to buy when they are in green but you will get little or no profit through that, more so you will lose out. Study the trend of the alts you are interested in, understand the the graph for a reasonable period, chances are that there will be a trend that will be followed over a period and that will give you some clue as when to jump into the ship and when to cash out before you get burnt.

Happy Trading