I think it's depend on your whole assets, it's would be 0.05 or 0.5 depend on how rich are you. Or maybe you could try to do the signature campaign and keep all the rewards for trading.
The amount of the money you want to invest really depends on how much you can afford. But as a starter, I suggest you have to invest a little amount first as a practice, when worse happens, then it would not be so big loss for you. Practice makes perfect, so as the time goes by, you learn more and gain and earn.