I was told by a friend of mine that there's a max percentage of CD reduction that actually takes effect. After looking around, the CD max is 25% apparently. Can someone confirm this or clear the cooldown reduction part of the system up for me? Trying to tweak a general build for a few Magic characters.

Any helpful feedback is greatly appreciated! Thanks! =)

40% is pseudo-hard cap. Future maps might not have it implemented! Look out for those as the actual hardcap is somewhere around 91%.

Way to be confusing MammothWaffle

@OP: For all intents and purposes, untill otherwise noted, only 40% cooldown reduction will have an effect on you. Do note that there are abilities (Zileans rewind, Ezreals Mystic Shot) which have further cooldown effects that can technically bring your cooldown "above" the cap.

Anyways, if your adding up CDR, its 40%, nothing more will affect you.

I love getting 40% CD and using intervention every 30 seconds

All CD reduction percentages stack additively.

So if you have:
6% from runes (you can get 5.85% from Tier 3 Flat CD reduction Glyphs)
9% from masteries (from a 9/0/21 setup)
10% from one item (say, Haunting Guise)
15% from the Golem buff (usually higher, but we'll assume 15% for the sake of simplicity)

then you have a total of 6+9+10+15=40% cooldown reduction, which is the maximum, making your spells cooldown in 60% of the time they would otherwise cooldown.

This means a 10 second cooldown is now a 6 second cooldown, a 60 second cooldown is now a 36 second cooldown, and a 4 second cooldown is now a 2.4 second cooldown. (LoL will break it into fractions of a second, probably rounding to some hundredth of a second.)