Is there a limit to how much cooldown you can reduce? For example I stacked Nashor's Tooth (25%), Frozen Heart (25%), Spirit Visage (12%), and Soul Shroud (15%) which should give me a total of 77% cooldown reduction. I did this in an attempt to play as an artillery with Gankplank but giving support fire with his ultimate, but the math didn't add up as it only shaved off 45 seconds on the cooldown making me wait 75 seconds instead of 120 seconds. Can someone explain this to me I was under the impression that through my hard work in item building I would reap the benifits of having his ultimate go off every 30 seconds.

There is a hard cap of 40%, whether the reductions come from items, buffs, or masteries.