As long as it needs to be. Seriously, when you're talking about the destruction and recreation of existence with time being a part of that existence, it becomes very hard to measure time between two Kalpas. The rules, after all, will change and time will flow differently and at a different pace. Or if you want it from a mythic perspective, a Kalpa is the time between when ol' Alduin has eaten last and becomes hungry again.
But that then leaves the questions of what do the JYLS do in that time? Are they dormant, or are they essentially the ones who rebuild? Also, wouldn't that make a Dragon Break much like a visceral gap between Kalpas? Because if every possible possibility happens in that time, couldn't a Kalpa pass, or a C0DA, or a Landfall, or even a inverse of the I Itself; with some admiration to the pre-existing structure of Scripture, seeing as I'd be pretty hard to Reprimand a cosmological identity with no fingerprints to go off of; happen, and then re-happen a billion more times before the ants below snap back into an indigenous existence?
And what if someone (a Godhead) pulls a Yagrum, and exits AE in that time? Do we know if anything like The Provisional House is affected? Because from what I remember (or wrote, whichever, I'm a little skank like that), looking down onto Nirn is like using Google Maps in there, so there'd have to be some sort of dimensional connection between them. You'd think a little bit of Alduin's digestive slime would seep in there...it? Eh, whatever, the space that isn't a space yet still contains a parliament of lines.