There are times when stock and crypto market is low, it was like that last year and when it is high (around year 2000 or beginning of 2021 if i remember correctly). Watching the long term graphs (i mean maximum time frame, ideally tens of years) and reading the news can give idea when market is overvalued and when it is reasonably valued (in the middle of crisis when you read headlines/articles how Bitcoin is "dangerously low"), it is time to buy. You can not guess perfect time. Only maximize your chances by buying low in crisis and selling high when everyone is positive and greedy during all time high prices (ATH). Selling crypto is more risky since crypto will gain value in long term.