If you're looking for a serious investment, you need to dollar-cost average and just make a regular weekly/monthly purchase of the same amount.
If you're just trying to speculate and make a quick buck, you need to make your own guess on where the value is going to go from here.
I bought $100 at around $23 and ended up using/losing all of it. I bought $100 more at around $31 and have been sitting on it since. Now at around $62 I've recouped ALL my bitcoin investment. It's really just luck and how much you're willing to invest.
Best advice is the same as gambling: Don't put in any more money than you can afford to lose.
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Thank you for advice.