24 second cooldown Zhonya's

Perfect Timing: 15% CDR for Gargoyle Stoneplate, Zhonya's Hourglass, and Guardian Angel

Cosmic Insight: 5% Item Active CDR

Ingenious Hunter: 40% Item Active CDR at full stacks

Ruby Sightstone: 20% Item Active CDR

Total Item Active CDR: 80%

All of these stack additively, I checked in-game to make sure.

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With all of these combined, you end up with an 18.4 second cooldown for Gargoyle Stoneplate and a 24 second cooldown for Zhonya's Hourglass. Guardian's Angel is only reduced to 255 seconds, since it isn't an active (but it would be 60 seconds, if the rest of the CDR affected it).

This is... mildly disgusting. Not that people are likely to ever build this, with Ruby Sightstone being a support item. Even without it, you have a 36.8 second cooldown for Gargoyle and a 48 second cooldown for Zhonya's. These are really low numbers.