Just look at the market and check whether the asset or coin you're eyeing is ripe enough to be bought. Meaning, if the price is low enough for you to carry and buy, then go for it, although it does matter that you get it for the lowest value possible so that you can get every bang for your buck; still, the one to decide whether it's a good time or not is none other than you, on the other hand, the time which is "best" to sell would be basically when you reach the margin of profit you want, every person sets a different bar, for most people and myself as well, I try to get 30 to 50% gain, if I reach that, then I would fee good enough about my investment, and so, selling would not be so bad.