Peace spells are not generally the type of spells which will bring peace on Earth but they do create more peaceful situation.
Generally a peace spell will create an aura of peace around you. All spirits moving near and though will will be at peace with themselves and the others around them. As time goes on this peaceful feeling spreads.
How far this spreads and how long the effect lasts is based partly on the strength of the caster, but more so on the level of peace of the caster. Usually once the caster becomes upset or angry the spell will be broken.