30 hours (~3 days)
This is one of those "I'll get it done by dinner" -projects, which then stretches and stretches because you are having too much fun coming up with features. Well, it is a simple app meant to exercise user's short-term memory and mental arithmetics. On the easy mode, it is all nice and fun. On the hard mode, your brains are in for a ride.
Numbers are moving through the screen and your tasks is to sum them all up. Quicker you do this, more points you get. On each round, some numbers are visible, some are hidden - so better not forget those which you can't see.
I still use this app every single morning for 10 mins - really puts your brains into "lets go" -mode. I think using this app for hours at a time may drive you into psychosis or something - haven't tried yet. Although I did make a pseudo-scientific experiment about effects of alcohol-consumption on mental computation speed.