It checks when the power is first triggered, and then rechecks once every 10 seconds for longer lasting powers. (Assuming that the effect that the proc is associated with is reapplied to the target.)
If Brevity is the Soul of Wit, Why are You Reading This?
Thanks, so it will basically have a chance to proc twice since it's duration is 15 seconds. If anything survives the rain/fireball/firebreath combo that is. 
Make a man a fire and keep him warm for the day, SET a man on fire and keep him warm for the rest of his life.
Incarnates: K'lir(Fire/Dark Corr):Hot-House Flower(Plant/Fire Dom):Kinrad X(Kin/Rad Def):Itsy-Bitsy Spider(Crab):Two Ton Tony(Mace/WP Broot):Teeny Weeny Widow(Fortunata/Widow) : Zeroth Law (Ice/Fire Tank)
If I put this proc in "Rain of Fire", how does the proc work?
I assume it has a chance to proc on anyone in the area, but does it go off every couple of seconds, or just once at the beginning?
Make a man a fire and keep him warm for the day, SET a man on fire and keep him warm for the rest of his life.
Incarnates: K'lir(Fire/Dark Corr):Hot-House Flower(Plant/Fire Dom):Kinrad X(Kin/Rad Def):Itsy-Bitsy Spider(Crab):Two Ton Tony(Mace/WP Broot):Teeny Weeny Widow(Fortunata/Widow) : Zeroth Law (Ice/Fire Tank)