I was doing a little math and found that, although I knew 100%cdr would give you 0 cd...

I never truly though much about that.

However I did some math and imagined a scenario...

Imagine a champ with no mana/energy/heat.

A champ with 0 casting time on all of their abilities.

This would mean that the effect of cdr would be more direct.

Imagine now, for simplicity that their skills are all 10 second cd...

Now take a 100second time to measure dps in.

Also just for simplicities sake imagine each skill does 1 true damage...

Now with 0 cdr, every 10 seconds he would do 1 damage... dealing 10 damage over the full 100 seconds.

100[timeframe] / skillcd = casts in 100 seconds...

Using this and the base damage you can figure how much dps change is going on.

Here is the result...

1%CDR GIVES +1%DPS

2%CDR GIVES +2%DPS

3%CDR GIVES +3%DPS

4%CDR GIVES +4.1%DPS

5%CDR GIVES +5.2%DPS

6%CDR GIVES +6.3%DPS

7%CDR GIVES +7.5%DPS

8%CDR GIVES +8.6%DPS

9%CDR GIVES +9.8%DPS

10%CDR GIVES +11%DPS

...

20%CDR GIVES +25%DPS

...

30%CDR GIVES +42%DPS

31%CDR GIVES +44%DPS

32%CDR GIVES +47%DPS

33%CDR GIVES +49%DPS

34%CDR GIVES +51%DPS

35%CDR GIVES +53%DPS

36%CDR GIVES +56%DPS

37%CDR GIVES +58%DPS

38%CDR GIVES +61%DPS

39%CDR GIVES +63%DPS

40%CDR GIVES +66%DPS

So the first couple of percents give about a 1-1%cdr to %dps increase.

However near the last 10 percent cdr, the dps increases go up to 2-3%.

Now remember this ignores casting times and skill costs, as those will penalize all cdr in a linear manner, essentially not effecting the curve of the dps boost.

This should tell you, that buying the first 10% cdr isn't that great.

But that last 10% is more than twice as good as the first 10%.

So in conclusion, if you're going to get ANY cdr, you may be better off maxing the cdr.

This counts for the mastery increases of 1-10%... and the runes which can increase the cdr even more.

Treat cdr like an addiction, if you're getting 1-5%, might as well get 40% XD