Perfect Defense is solely based on the Summoning Magic Skill now. You can call Alexander with 1 MP and 520 smn skill, PD will last 56 seconds. The decay effect is still present but there is something very strange about it.
On the live server PD start to decay at half of the duration (i.e. 45 seconds of DT-96% with the max duration of 90 seconds)
On the Test Server, if I uses Perfect Defense with 100% MP (or probably just more than 1188 MP), the effect start to decay after 28~30 seconds.
However, if I uses Perfect Defense with less than 100 MP it start to decay after 50~52 seconds, that's a difference of 20 seconds.
I made these tests many times using Fraps/Timestamp and the results were practically the same on every occasion, it's not random at all.