The best is when you're financially ready, no time is really the best time
The best time would be then when you are ready to do it without the risk for your other endeavors.
In that case, you are just building up what will benefit you for the years to come
This does not seem like the right answer because you don't get ready to buy Bitcoin, you buy Bitcoin when you have discretionary income to out into Bitcoin, such that you can hold for many years and not be in a hurry to sell. You can start with as little as $10 if that is truly your discretionary income but it would be a mistake waiting till you have big amount of money because that might make you never get started.