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.

-hashcode