The question of when to buy is best answered based on your investment goal. The time the Mr Bob would want to buy will not be the time that Mrs Alice will want to buy if both has different investment goals.
For instance, bitcoin price is within $37k and anyone who is willing to take profit in the next 2 years can buy bitcoin now and wait till the next 2 years and they will be on profits.
There are people who will buy at 37 because they are targeting 40k, which I think is somewhat risky. While some buy continuously with the DCA method.
So, everyone has their entry points according to their investment goals but the best rule to buy is to buy low and sell high.