I think the right time to buy coins is in exchange, because we can already monitor the price. usually the price will dump when a few days there is a new coin enter the market, where we can buy coins and we hold for a long time, to wait for prices to rise again. we can also buy Coins time Ico, because usually we can get bonus when we buy coins time of ICO.
Sometimes going to the exchange is an advantageous time to buy, but more often than not, the biggest profit comes from pre-sale. But several times I saw that the price of the coin fell below the price of ICO and it was a very advantageous offer for purchase.