For ease sake, let's say I got a Ult that has exactly 100 second CD. I have 40% CDR making it now 60 seconds. I also have the +5% from runes bringing it to 55 seconds. Now I have 5 Stacks on Ultimate Hunter giving me 25% more bringing it now to 30 seconds. And now hypothetically speaking I was able to get 4 Cloud Drakes getting 40% more. Does that just make the cool down on my Ult to 0 seconds?
Item CDR stacks additively with itself and Cosmic Insight. Ultimate Hunter stacks multiplicatively (additively with itself). The cloud dragon buff stacks multiplicatively (additively with itself). The max CDR for ultimates is 75.25%.
No
because I don't think the CDR is always going off the original value
I did a test with 
because at lvl 16 his ult CD is 100 [no items or runes included]
now since Ult Hunter gives you 5% off your Ult by default and you get 5% CDR from Cosmic Insight, your Ult technically starts with a 10% CDR on it
His ult, at that level with those two CDR boosts in place; has a total cd of 90.25s
with the added 40% CDR from various items, his Ult drops down to 52.25
10% First Cloud Drake = 46.75 10% Second Cloud Drake= 41.25 10% Third Cloud Drake = 35.75 10% Fourth Cloud Drake = 30.25
Ult Hunter can gain 5 stacks at 4% each
Now I can only get 4% with the Practice Tool, but that will give me the number to go off of to get the rest
One stack of Ult Hunter = 28.05
Please note the other 4 stacks will go off the new value I get rather than the original 100 CD value The new value may be a bit off compared to official Riot math but it is likely the best you will get
2nd = 26.93 3rd = 25.86 4th = 24.83 5th = 23.84
Now someone who works from Riot is more than welcome to check this and correct it if I'm wrong But I did the best I could given the circumstances and limitations of League (i.e. not being able to guarantee 4 Cloud Drakes to spawn in a normal match and not being able to have more than 1 bot in the practice tool to see exact number values off the last known value)
