There is no hard and fast rule for that. The general rule is, buy at dips and sell at high. However, you need to track the market very closely before you know when to buy and when to sell. There is no rule or checklist which can tell you the exact time. That comes with experience. Also patience is a must have quality for any trader. If you are new in trading, try youtube. Lots of videos are available to learn trading. Start with small amount and as you grow confidence, gradually increase the amount.
Very accurate! I would also add the following: never trust people on the internet telling you X coin is going to pump/dump you might get burned. You have to use your guts to determine when is the right time to get in/get out