I got 52% after a good lot of keypresses (not sure exactly how many). I chose to not look at the screen at all during that time - I think that probably helps.
Mine started at 25% and worked it's way up to 54%. Which also involved me holding down keys for a few seconds. I'm not sure if it works or if Hacker New's user base is skewed towards people who are slightly better at being random. (or if that's cause by self reporting, would someone whose best was 80% want to come forward?)
Edit:
Interesting tidbit: I pre-seeded at 100% by holding down F and it trended down to ~68% before I decided I shouldn't spend too much time on this. Maybe we didn't use it long enough?
There is an inherent 50% probability of each character occurring next. i.e if you were to guess randomly, you would be right 50% of the time given enough trials.
