Pretty sure the invincibility is instant, he can interrupt Scorpion's 334 with it which has a 2 frame gap after the 2nd 3. I'm not sure if I'd recommend it anyway for all the B1xxlolnonomaddashsonnnnnnn Kabals out there lol.
b1*21 - 4 frames
The gap is between the first and second attack.
It can be interrupted somewhat easily. (e.g. Pig of the Hut is able to armor through consistently)