How long can a hen leave her eggs for?

If you mean during the time in which she is trying to hatch chicks, then the maximum amount in which she can leave her eggs for a quick break is about 1-5 hours at most, depending upon the temperature and surroundings where the eggs are. The easiest way to judge is by letting your chicken have a break and continually checking the eggs she left behind for warmth. Once those eggs have lost their warmth, then the hen must come back within the next hour to warm them up again.