While experts don't all agree on the ideal humidity level for hatching eggs, it is generally recommended that the level should not go under 25% or go over 60% between the day you first set the eggs into the incubator and three days before the estimated hatching day. Experts recommend that you add extra water three days prior to hatching so the humidity levels go to 70 to 80%. Most incubators come with their own instructions about how often to add water, so it's always a good idea to follow those instructions.