This has an internal cooldown of 45s, starting upon proc.
Effective Spellpower
The effective spellpower granted by this trinket is calculated as a function of cast-time. Essentially, to get the most out of this trinket, you want shorter cast times.
The formula for calculating the amount of effective +damage granted by this item is -
E = 850 * 10 / (45 + (t / 0.1)))
Where...
E = effective +damage
t = cast time
The following Java Program can be used to obtain a table of effective +damage for different casting speeds: