for me, it doesn't matter that much on when and where. i have worked with the strike from starting point all the way to almost hitting me and anywhere in between. it's a game i play actually just to see if i don't bang the arms, still disrupt uke's balance, and stay relax at the same time. my favorite spot lately has been waiting for the strike to almost hit my head, then bring my arm up to deflect and not moving my feet at all. sort of a wingchun version then follow with a shiho nage or whatever depends on uke's balance. of course, now folks would scream "That's not aikido". my answer would be "that's my aikido. go away!".
"let me show you that footwork again..."
With my bum knee I'm doing as little footwork as I can get away with. Means I sometimes come close to getting clipped by a yokomenuchi or a munetskuki but I'm willing to risk it to learn the margins.
I don't know if I'd call what I'm aiming to do an "interruption" as much as a momentary disruption - both George Ledyard Sensei and Chuck Clark Sensei talk about ways of creating disruption in a way that doesn't make uke either root or immediately change attacks, creating an opening. I find a lot to play with in there....