This post isn't really complete, but I accidentally teleported myself and am done for now, so want to get the information recorded somewhere in case I close the text file I'm documenting things in.
Aquaveil mechanics:
First, I tried to make Aquaveil wear off on RDM without casting. It doesn't. I took 7000 damage (10 minutes of getting beaten on) and it didn't wear off. You need to be hit while casting to make Aquaveil wear off.
Next, I switched to NIN/WHM and went out to find a level 100 Chapuli.
Method:
Code:
* Cast Aquaveil.
* Cast Teleport-Holla and move.
* Keep track of how many times you're hit during Teleport-Holla until Aquaveil wears off and record it.
* Repeat x20.
1 hit - 12 times
2 hits - 5 times
3 hits - 3 times
This works out to be a 40% chance of it wearing off every time I'm hit. I either didn't see a 4-hit case because I got unlucky or because it's capped at 2 extra hits.
I changed job to SMN/WHM and repeated the test, this time with Vadose Rod, Shedir Seraweels, and Emphatikos Rope. I accidentally teleported myself after 10 attempts, but:
4 hits - 5 times
5 hits - 3 times
6 hits - 2 times
Approximately the same distribution, but shifted up by 3 hits.
It is worth noting that in the above tests despite having subjob magic skill I did not have a 100% interruption rate. It is possible that the 1~3 range and 4~6 range are due to non-interruptions, and Aquaveil is just absorbing a static number of interruptions. This should become obvious with a larger sample size for the SMN case, because you wouldn't expect it to always be a 2-hit range as you got to longer durations of aquaveil.
Next steps would be to:
1) Complete the SMN test (another 10 trials)
2) Switch to a job with 500 enhancing skill and see how that affects the distribution without enhancing items (WHM/SCH with Light Arts up casting Klimaform?).
3) Add Aquaveil+ items and check for a cap.
4) Maybe test a middle point (like BLM's enhancing skill) to get some idea about the progression.