It's up to the personal financial state and commitment; greatly depends on how well off the investor is.
We all have to agree with the fact that Bitcoin is a risky investment. With the high return, it comes with its risk. Hence I'd pick the first option for everyone ("how much you can easily invest without stress"). If you put all-in to Bitcoin, you'll have my respect mate. But most of us needs stable currency for our day to day usage.
Well, as much as we know how risky bitcoin investment is, then we should always invest on the amount we can afford to lose. And when that investment prosper, we can start to adjust our investment capital and increase our efforts in it. But no matter how profitable bitcoin is, it’s still advisable to invest with caution and never let your emotions decide when to sell your coins because most likely, if you are feeling negative emotions, selling at its wrong timing will only bring you a lot of losses.