Just as much as you can invest without cutting your daily ration and everyday needs, we possess an unbelievable possibility to create our wealth for future and we should use it lively.
Investment in bitcoin is actually not something to be measured with the amount of money. It's more like adapting the idea of blockchain. Even $100 or more, this doesn't change the fact that you trust in bitcoin.
So there is no fixed amount you should invest in bitcoin. This is up to your financial situation.