When exactly do you have invulnerability with the Long Sword ?

[Vakula]

Specifically with the counter RT + B and Iai slash + Iai Spirit slash? For the RT+B basegame counter its just when you glow blue? and for the Iai if you attack just as the monster attacks you dont get damaged? Is that correct?

[Tiom]

In iceborne you need to have the spirit gauge slightly charged before foresight slash (RT+B) will give you i-frames and the very satisfying "ding". You are correct for the Iai attack, however, some monsters attacks/movement have long hitboxes and the i-frames don't last long enough to keep you from harm. Iai i-frames are very shot compared to foresight slash.

Edit: I found a video that i watched a while back. It explains i-frames for different dodges and weapon moves in the first 2 minutes.

https://www.youtube.com/watch?v=cmhPbtpH6Ts