It's almost instantaneous, so it's quite a bit of time. No idea on how many frames, though. You do have to be in the air, and airdashing, before you activate nirvana, so after those frames (however long activation takes,) you can FD.
Hmmm.. I've only thought of it as a defensive option, but this is an interesting idea.
You want that to combo instead of reset, right? I'll try, but you'd have to activate nirvana out of the jump canceled 5C>IAD. (So I'm not sure how it'd work..)
I'll make a rough recording (handy cam type thing "/) of what it looks like just doing naked IAD 4D>FD.
It seems like him activating nirvana cancels the airdash animation, when he lifts his arm and such.
Pretty cool stuff. Might be kind of impractical, but just a weird thing I found.