Offensive spells will benefit from every point in intellect, defensive and booster spells usually benefit from duration and intellect (if int is a factor in the calculation formula).
For example, stone skin, dispel and such have no intellect factor in the formula. God Armor, Healing, damage amplifying spells versus enemies (demons, dragons, etc.) have intellect as a factor in the formula.
I think this can be summarized like this:
1. Offensive spells benefit from every intellect point.
2. Spells that carry certain amount of power (heal, god armor, etc.) also benefit from intellect. Still, there are exceptions.
3. Spells with instant effect or spells with fixed power (like stone skin, calm rage, dispel, haste, etc.) do not benefit from every point of intellect except for duration (if there is one).
I can give you precise formulae for every spell, but writing that down will take too much space and too much time
use the basic principle I've written above and you'll be just fine. You may also want to check the forums for the translated version of the KBAP manual.