The best advice I've seen is: Don't invest more than you can afford to completely lose.
I always have a tough time calculating how much I
really can afford to lose.
I might get diagnosed with cancer tomorrow and then I might need every penny of my savings. When I think about it that way, even $100 seems to much to invest in Bitcoin.
On the other hand, if I lose $10,000 on Bitcoin, perhaps I won't be able to afford a few luxuries for a few months. Not that big of a deal.