This skill is automatically checked when a caster casts a spell with a verbal component. Each round of casting time is checked up to half the casting time.
If a successful check is made that round is removed and casting time reduced.
Thus a person with 100% quick chant will have 1/2 casting time as they will succeed each time check up to half. A person with 50% quick chant will succeed only half the checks on average and result in a 1/4 reduction in casting time. Note that quick chant does not work on spells of the charm, psionic, sonic, conjuration, or invocation category.