The Duration entry of a spell description tells how long the effect of the spell lasts.
Timed Durations: Many durations are measured in rounds, minutes, hours, or some other increment. When the time is up, the magical energy goes away and the spell ends. If a spell’s duration is variable, the GM rolls it secretly.
Instantaneous: The spell energy comes and goes the instant the spell is cast, though the consequences of the spell might be long-lasting.
Permanent: The effect remains indefinitely, but is sustained by lingering magical energy. If the energy goes away, so does the effect.
Concentration: The spell or power lasts as long as the caster concentrates on it, possibly up to a specified maximum amount of time. Concentrating to maintain a spell is an attack action that does not provoke attacks of opportunity. Anything that could break the character’s concentration when casting a spell can also break his or her concentration while maintaining one, causing the spell to be ruined (see Concentration). A character can’t cast a spell while concentrating on another one.
Sometimes a spell lasts for a short time after the character ceases concentrating. In these cases, the spell keeps going for the stated length of time after the character stops concentrating.
Subjects, Effects, and Areas: If a spell affects creatures directly, the result travels with the subjects for the spell’s duration. If the spell creates an effect, the effect lasts for the duration. The effect might move or remain still. Such an effect can be destroyed prior to the end of its duration. If the spell affects an area, the spell stays with that area for the spell’s duration. Creatures become subject to the spell when they enter the area and are no longer subject to it when they leave.
Discharge: A few spells last for a set duration or until triggered or discharged. The spell remains in place until the triggering condition is met (at which point it takes effect) or the maximum duration is reached (at which point it dissipates, with no effect).
(D): If the Duration entry ends with “(D)” (standing for “dismissible”), the caster can dismiss the spell at will. The caster must be within range of the effect of the spell to dismiss it. Dismissing a spell is an attack action that does not provoke attacks of opportunity. A spell that depends on concentration is dismissible by its very nature, and dismissing it does not require an action (since all the caster has to do to end the spell is to stop concentrating).