It seems like rekka 2 is around -17 when not delayed at all. If you put Mileena on reversal EX roll, you'll get hit.
What's interesting is that when you do the no gap medium delay, that previously made you -4 if you complete all the rekkas and was patched out, you actually knock off a few frames when you stagger rekka 2. I'm able to block Alien's snag which is 14 frames.
You can block moves like Mileena's roll, Alien's snag, Jin's cartwheel, and even Jax's overhead dash punch. The latter is huge because I personally view Jax as one of Alien's hardest match ups.
Of course a good player will just punish you outright with a normal if you do this consistently, however, it's become common knowledge that you can option select Alien's rekkas with a lot of different armored moves. If you do this, you can definitely bait some of those in a tournament set.