Why are typically fatal currents non-lethal when they pass through your body for a short time?
You need to bear in mind that there are many different modes of damage possible,


Large current passed though tissue may burn the tissue
Modest current passed through a vital muscle (like that of the heart) may cause that muscle to operate in a manner that it's not used to and strain it so that it is damaged from the strain. 
Very tiny current may interfere with the normal nerve signals that drive the heart's operation, throwing off the heart rhythm leading to ineffective pumping for long enough to be fatal.


So it's not a matter of stating a particular voltage, current, charge, energy or power. It's a matter of stating a range of very particular and quite different circumstances that pertain to different body systems.