That’s hard to tell because nobody knows what the price of Bitcoin will be in future. Now, the price of Bitcoin is very low, so it is a good time to invest. But it is hard to tell when the price of Bitcoin will recover so invest if you can take the risk.
So in this way, you can invest anytime in bitcoins if you want to be a long term holder. This is not as easy as one might think because this market is very volatile. Imagine that being able to consistently invest $50 per month over the next few years would definitely be the best way to generate a commensurate return compared to you investing in gold.
The best time is when you are very sure to buy bitcoin because you realize that there is good potential in the future. Currently the price of bitcoin is still quite low compared to ATH late last year, so do a little research and be brave to make a decision on your investment.