Tag said: "It's about 15 seconds"
FWIW, the number I've heard is 20 seconds, but it might depend on the production/actor, or even vary from performance to performance.
It's been a while since I've read Felicia Ricci's book about being the Elphaba standby, so I don't remember if she talked about this. But it's the kind of thing she covered in her memoir (I've recommended her memoir "Unnaturally Green" a few times before - I really love it, though I'm also biased because I saw the SF production of Wicked several times, including with Ricci as Elphaba).
As for whether it's the shortest quick change, it sort of depends on what you count. A lot of quick-changes can be accomplished in a split second if it's just a tear-away costume. Or just a few seconds onstage like the transforming costumes in Cinderella. But is it the shortest amount of time relative to the complexity of the quick-change? Maybe, but it's hard to measure that without a way to measure how complex other quick changes are.
But suffice ti to say: it's impressive!