The perfect time to invest is when you're ready. As you've mentioned, it is hard to predict the lowest market price Bitcoin could reach and that is true. And that is also why investing once you can afford to set aside such amount, is the perfect time. Once you've entered this market, you must be equipped with the idea of loss possibilities. The market price could either go up or continue to fall down once you invested, and that would be the problem if you are just hoping to earn profit as an investor. Right now, there are still no clear signs of market recovery and it could be months or year before such thing happen; no one knows. So if you plan to invest, could you endure?
A few weeks ago the market was dominated by negative news, but today the situation is changing and we see that the price is turning up. Who could predict what the movement would be, or who can say what will happen in a day, a week or a month? That's right, no one has such information and therefore you need to buy at regular intervals, say every week, or every month, as you like. Determine the amount that will be convenient for you and start your way as an investor.
Imagine a situation where a person only saves money but does not buy, he is waiting for a better time. He waited when the price dropped to 15500, waited when the price was 16500 and continues to wait now, when it almost reached 18000, and what will happen if he waits further but in a week the price will already be 20k, and in a month 30k, at what level will he start buying? And he was waiting for this all the time, because many people said that the price could drop to 10k?
Now let's imagine that he bought every week at all levels for an equal amount, say $ 100, in which case he gets the average price and does not worry about when to buy. He has an investment with a good average purchase price. So don't wait for the perfect time to invest, just act.